diff --git a/anonstream/__init__.py b/anonstream/__init__.py
index 3bedfa4..39e2082 100644
--- a/anonstream/__init__.py
+++ b/anonstream/__init__.py
@@ -3,6 +3,7 @@
 
 import asyncio
 import json
+import os
 from collections import OrderedDict
 
 from quart_compress import Compress
@@ -11,6 +12,7 @@ from anonstream.config import update_flask_from_toml
 from anonstream.emote import load_emote_schema
 from anonstream.quart import Quart
 from anonstream.utils.captcha import create_captcha_factory, create_captcha_signer
+from anonstream.utils.locale import validate_locale, Nonconforming
 from anonstream.utils.user import generate_blank_allowedness
 
 __version__ = '1.6.6'
@@ -54,6 +56,22 @@ def create_app(toml_config):
     except (OSError, json.JSONDecodeError) as e:
         raise AssertionError(f'couldn\'t load emote schema: {e!r}') from e
 
+    # Read locales
+    app.locales = {}
+    for lang in app.config['LOCALE_OFFERED']:
+        filepath = os.path.join(app.config['LOCALE_DIRECTORY'], f'{lang}.json')
+        with open(filepath) as fp:
+            locale = json.load(fp)
+            try:
+                validate_locale(locale)
+            except Nonconforming as e:
+                error, *_ = e.args
+                assert False, f'error in locale {lang!r}: {error}'
+            else:
+                app.locales[lang] = locale
+    app.lang = app.config['LOCALE_DEFAULT']
+    app.locales[None] = app.locales[app.lang]
+
     # State for tasks
     app.users_update_buffer = set()
     app.stream_title = None
diff --git a/anonstream/chat.py b/anonstream/chat.py
index 667f591..2bbad1b 100644
--- a/anonstream/chat.py
+++ b/anonstream/chat.py
@@ -50,34 +50,24 @@ def add_chat_message(user, nonce, comment, ignore_empty=False):
         user['linespan'],
     ))
     if total_recent_linespan > CONFIG['FLOOD_LINE_THRESHOLD']:
-        raise Rejected(
-            f'Chat overuse in the last '
-            f'{CONFIG["FLOOD_LINE_DURATION"]:.0f} seconds'
-        )
+        raise Rejected('message_ratelimited', CONFIG['FLOOD_LINE_THRESHOLD'])
 
     # Check message
     message_id = generate_nonce_hash(nonce)
     if message_id in MESSAGES_BY_ID:
-        raise Rejected('Discarded suspected duplicate message')
+        raise Rejected('message_suspected_duplicate')
     if len(comment) == 0:
-        raise Rejected('Message was empty')
+        raise Rejected('message_empty')
     if len(comment.strip()) == 0:
-        raise Rejected('Message was practically empty')
+        raise Rejected('message_practically_empty')
     if len(comment) > CONFIG['CHAT_COMMENT_MAX_LENGTH']:
-        raise Rejected(
-            f'Message exceeded {CONFIG["CHAT_COMMENT_MAX_LENGTH"]} chars'
-        )
+        raise Rejected('message_too_long', CONFIG['CHAT_COMMENT_MAX_LENGTH'])
 
     if comment.count('\n') + 1 > CONFIG['CHAT_COMMENT_MAX_LINES']:
-        raise Rejected(
-            f'Message exceeded {CONFIG["CHAT_COMMENT_MAX_LINES"]} lines'
-        )
+        raise Rejected('message_too_many_lines', CONFIG['CHAT_COMMENT_MAX_LINES'])
     linespan = get_approx_linespan(comment)
     if linespan > CONFIG['CHAT_COMMENT_MAX_LINES']:
-        raise Rejected(
-            f'Message would span {CONFIG["CHAT_COMMENT_MAX_LINES"]} '
-            f'or more lines'
-        )
+        raise Rejected('message_too_many_apparent_lines', CONFIG['CHAT_COMMENT_MAX_LINES'])
 
     # Record linespan
     linespan_tuple = (timestamp, linespan)
diff --git a/anonstream/config.py b/anonstream/config.py
index e9f842f..ce4275b 100644
--- a/anonstream/config.py
+++ b/anonstream/config.py
@@ -40,6 +40,7 @@ def toml_to_flask_sections(config):
         toml_to_flask_section_captcha,
         toml_to_flask_section_nojs,
         toml_to_flask_section_emote,
+        toml_to_flask_section_locale,
     )
     for toml_to_flask_section in TOML_TO_FLASK_SECTIONS:
         yield toml_to_flask_section(config)
@@ -171,3 +172,12 @@ def toml_to_flask_section_emote(config):
   return {
     'EMOTE_SCHEMA': cfg['schema'],
   }
+
+def toml_to_flask_section_locale(config):
+  cfg = config['locale']
+  assert cfg['default'] in cfg['offered']
+  return {
+    'LOCALE_DEFAULT': cfg['default'],
+    'LOCALE_OFFERED': cfg['offered'],
+    'LOCALE_DIRECTORY': cfg['directory'],
+  }
diff --git a/anonstream/locale.py b/anonstream/locale.py
new file mode 100644
index 0000000..078935a
--- /dev/null
+++ b/anonstream/locale.py
@@ -0,0 +1,18 @@
+from quart import current_app
+
+LOCALES = current_app.locales
+
+def get_lang_and_locale_from(context):
+    lang = context.args.get('lang')
+    locale = LOCALES.get(lang)
+    if locale is None:
+        lang, locale = None, LOCALES[None]
+    return lang, locale
+
+def get_lang_from(context):
+    lang, locale = get_lang_and_locale_from(context)
+    return lang
+
+def get_locale_from(context):
+    lang, locale = get_lang_and_locale_from(context)
+    return locale
diff --git a/anonstream/routes/core.py b/anonstream/routes/core.py
index d1ea26c..6a1c36e 100644
--- a/anonstream/routes/core.py
+++ b/anonstream/routes/core.py
@@ -9,6 +9,7 @@ from werkzeug.exceptions import Forbidden, NotFound, TooManyRequests
 
 from anonstream.access import add_failure, pop_failure
 from anonstream.captcha import get_captcha_image, get_random_captcha_digest
+from anonstream.locale import get_lang_and_locale_from, get_lang_from, get_locale_from
 from anonstream.segments import segments, StopSendingSegments
 from anonstream.stream import is_online, get_stream_uptime
 from anonstream.user import watching, create_eyes, renew_eyes, EyesException, RatelimitedEyes, TooManyEyes, ensure_allowedness, Blacklisted, SecretClub
@@ -20,33 +21,39 @@ from anonstream.wrappers import with_timestamp
 
 CAPTCHA_SIGNER = current_app.captcha_signer
 STATIC_DIRECTORY = current_app.root_path / 'static'
+LANG = current_app.lang
 
 @current_app.route('/')
 @with_user_from(request, fallback_to_token=True, ignore_allowedness=True)
 async def home(timestamp, user_or_token):
+    lang, locale = get_lang_and_locale_from(request)
     match user_or_token:
         case str() | None as token:
             failure_id = request.args.get('failure', type=int)
+            failure = pop_failure(failure_id)
             response = await render_template(
                 'captcha.html',
                 csp=generate_csp(),
                 token=token,
+                locale=locale['anonstream']['captcha'],
                 digest=get_random_captcha_digest(),
-                failure=pop_failure(failure_id),
+                failure=locale['anonstream']['internal'].get(failure),
             )
         case dict() as user:
             try:
                 ensure_allowedness(user, timestamp=timestamp)
             except Blacklisted:
-                raise Forbidden('You have been blacklisted.')
+                raise Forbidden(locale['anonstream']['error']['blacklisted'])
             except SecretClub:
                 # TODO allow changing tripcode
-                raise Forbidden('You have not been whitelisted.')
+                raise Forbidden(locale['anonstream']['error']['not_whitelisted'])
             else:
                 response = await render_template(
                     'home.html',
                     csp=generate_csp(),
                     user=user,
+                    lang=lang or LANG,
+                    locale=locale['anonstream']['home'],
                     version=current_app.version,
                 )
     return response
@@ -54,27 +61,22 @@ async def home(timestamp, user_or_token):
 @current_app.route('/stream.mp4')
 @with_user_from(request)
 async def stream(timestamp, user):
+    locale = get_locale_from(request)['anonstream']['error']
     if not is_online():
-        raise NotFound('The stream is offline.')
+        raise NotFound(locale['offline'])
     else:
         try:
             eyes_id = create_eyes(user, tuple(request.headers))
         except RatelimitedEyes as e:
             retry_after, *_ = e.args
-            error = TooManyRequests(
-                f'You have requested the stream recently.  '
-                f'Try again in {retry_after:.1f} seconds.'
-            )
+            error = TooManyRequests(locale['ratelimit'] % retry_after)
             response = await current_app.handle_http_exception(error)
             response = await make_response(response)
             response.headers['Retry-After'] = math.ceil(retry_after)
             raise abort(response)
         except TooManyEyes as e:
             n_eyes, *_ = e.args
-            raise TooManyRequests(
-                f'You have made {n_eyes} concurrent requests for the stream. '
-                f'End one of those before making a new request.'
-            )
+            raise TooManyRequests(locale['limit'] % n_eyes)
         else:
             @with_timestamp(precise=True)
             def segment_read_hook(timestamp, uri):
@@ -112,6 +114,7 @@ async def captcha(timestamp, user_or_token):
 @current_app.post('/access')
 @with_user_from(request, fallback_to_token=True, ignore_allowedness=True)
 async def access(timestamp, user_or_token):
+    lang = get_lang_from(request)
     match user_or_token:
         case str() | None as token:
             form = await request.form
@@ -119,16 +122,16 @@ async def access(timestamp, user_or_token):
             answer = form.get('answer', '')
             match check_captcha_digest(CAPTCHA_SIGNER, digest, answer):
                 case Answer.MISSING:
-                    failure_id = add_failure('Captcha is required')
+                    failure_id = add_failure('captcha_required')
                 case Answer.BAD:
-                    failure_id = add_failure('Captcha was incorrect')
+                    failure_id = add_failure('captcha_incorrect')
                 case Answer.EXPIRED:
-                    failure_id = add_failure('Captcha has expired')
+                    failure_id = add_failure('captcha_expired')
                 case Answer.OK:
                     failure_id = None
                     user = generate_and_add_user(timestamp, token, verified=True)
             if failure_id is not None:
-                url = url_for('home', token=token, failure=failure_id)
+                url = url_for('home', token=token, lang=lang, failure=failure_id)
                 raise abort(redirect(url, 303))
         case dict() as user:
             pass
diff --git a/anonstream/routes/error.py b/anonstream/routes/error.py
index d82a930..02f55a2 100644
--- a/anonstream/routes/error.py
+++ b/anonstream/routes/error.py
@@ -1,8 +1,17 @@
-from quart import current_app, render_template
-
+from quart import current_app, render_template, request
 from werkzeug.exceptions import default_exceptions
 
+from anonstream.locale import get_locale_from
+
 for error in default_exceptions:
     async def handle(error):
-        return await render_template('error.html', error=error), error.code
+        locale = get_locale_from(request)['http']
+        error.description = locale.get(error.description)
+        return (
+            await render_template(
+                'error.html',
+                error=error,
+                locale=locale,
+            ), error.code
+        )
     current_app.register_error_handler(error, handle)
diff --git a/anonstream/routes/nojs.py b/anonstream/routes/nojs.py
index e34c52e..08e4dd6 100644
--- a/anonstream/routes/nojs.py
+++ b/anonstream/routes/nojs.py
@@ -5,6 +5,7 @@ from quart import current_app, request, render_template, redirect, url_for, esca
 
 from anonstream.captcha import get_random_captcha_digest_for
 from anonstream.chat import add_chat_message, Rejected
+from anonstream.locale import get_locale_from
 from anonstream.stream import is_online, get_stream_title, get_stream_uptime_and_viewership
 from anonstream.user import add_state, pop_state, try_change_appearance, update_presence, get_users_by_presence, Presence, verify, deverify, BadCaptcha, reading
 from anonstream.routes.wrappers import with_user_from, render_template_with_etag
@@ -12,7 +13,6 @@ from anonstream.helpers.chat import get_scrollback
 from anonstream.helpers.user import get_default_name
 from anonstream.utils.chat import generate_nonce
 from anonstream.utils.security import generate_csp
-from anonstream.utils.user import concatenate_for_notice
 
 CONFIG = current_app.config
 USERS_BY_TOKEN = current_app.users_by_token
@@ -25,6 +25,7 @@ async def nojs_stream(timestamp, user):
         csp=generate_csp(),
         user=user,
         online=is_online(),
+        locale=get_locale_from(request)['anonstream']['stream'],
     )
 
 @current_app.route('/info.html')
@@ -37,6 +38,7 @@ async def nojs_info(timestamp, user):
         {'csp': generate_csp()},
         refresh=CONFIG['NOJS_REFRESH_INFO'],
         user=user,
+        locale=get_locale_from(request)['anonstream']['info'],
         viewership=viewership,
         uptime=uptime,
         title=await get_stream_title(),
@@ -53,6 +55,7 @@ async def nojs_chat_messages(timestamp, user):
         refresh=CONFIG['NOJS_REFRESH_MESSAGES'],
         user=user,
         users_by_token=USERS_BY_TOKEN,
+        locale=get_locale_from(request)['anonstream']['chat'],
         messages=get_scrollback(current_app.messages),
         timeout=CONFIG['NOJS_TIMEOUT_CHAT'],
         get_default_name=get_default_name,
@@ -73,6 +76,7 @@ async def nojs_chat_users(timestamp, user):
         {'csp': generate_csp()},
         refresh=CONFIG['NOJS_REFRESH_USERS'],
         user=user,
+        locale=get_locale_from(request)['anonstream']['chat'],
         get_default_name=get_default_name,
         users_watching=users_by_presence[Presence.WATCHING],
         users_notwatching=users_by_presence[Presence.NOTWATCHING],
@@ -85,12 +89,14 @@ async def nojs_chat_form(timestamp, user):
     state_id = request.args.get('state', type=int)
     state = pop_state(user, state_id)
     prefer_chat_form = request.args.get('landing') != 'appearance'
+    print(state)
     return await render_template(
         'nojs_chat_form.html',
         csp=generate_csp(),
         user=user,
-        state=state,
         prefer_chat_form=prefer_chat_form,
+        state=state,
+        locale=get_locale_from(request)['anonstream'],
         nonce=generate_nonce(),
         digest=get_random_captcha_digest_for(user),
         default_name=get_default_name(user),
@@ -124,10 +130,10 @@ async def nojs_submit_message(timestamp, user):
     try:
         verification_happened = verify(user, digest, answer)
     except BadCaptcha as e:
-        notice, *_ = e.args
+        string, *args = e.args
         state_id = add_state(
             user,
-            notice=notice,
+            notice=[(string, args)],
             comment=comment[:CONFIG['CHAT_COMMENT_MAX_LENGTH']],
         )
     else:
@@ -143,10 +149,10 @@ async def nojs_submit_message(timestamp, user):
             )
             message_was_added = seq is not None
         except Rejected as e:
-            notice, *_ = e.args
+            string, *args = e.args
             state_id = add_state(
                 user,
-                notice=notice,
+                notice=[(string, args)],
                 comment=comment[:CONFIG['CHAT_COMMENT_MAX_LENGTH']],
             )
         else:
@@ -185,13 +191,13 @@ async def nojs_submit_appearance(timestamp, user):
     # Change appearance (iff form data was good)
     errors = try_change_appearance(user, name, color, password, want_tripcode)
     if errors:
-        notice = Markup('<br>').join(
-            concatenate_for_notice(*error.args) for error in errors
-        )
+        notice = []
+        for string, *args in (error.args for error in errors):
+            notice.append((string, args))
     else:
-        notice = 'Changed appearance'
+        notice = [('appearance_changed', ())]
 
-    state_id = add_state(user, notice=notice, verbose=len(errors) > 1)
+    state_id = add_state(user, notice=notice)
     url = url_for(
         'nojs_chat_form',
         token=user['token'],
diff --git a/anonstream/static/anonstream.js b/anonstream/static/anonstream.js
index 1625afb..4498299 100644
--- a/anonstream/static/anonstream.js
+++ b/anonstream/static/anonstream.js
@@ -12,16 +12,16 @@ const CSP = document.body.dataset.csp;
 
 /* insert js-only markup */
 const jsmarkup_stream_video = '<video id="stream__video" autoplay controls></video>'
-const jsmarkup_stream_offline = '<header id="stream__offline"><h1>[offline]</h1></header>'
+const jsmarkup_stream_offline = '<header id="stream__offline"><h1 data-string="offline">[offline]</h1></header>'
 const jsmarkup_info = '<div id="info_js" data-js="true"></div>';
 const jsmarkup_info_float = '<aside id="info_js__float"></aside>';
-const jsmarkup_info_float_button = '<button id="info_js__float__button" accesskey="r">Reload stream</button>';
+const jsmarkup_info_float_button = '<button id="info_js__float__button" accesskey="r" data-string="reload_stream">Reload stream</button>';
 const jsmarkup_info_float_viewership = '<div id="info_js__float__viewership"></div>';
 const jsmarkup_info_float_uptime = '<div id="info_js__float__uptime"></div>';
 const jsmarkup_info_title = '<header id="info_js__title"></header>';
 const jsmarkup_chat_messages = `\
 <ol id="chat-messages_js" data-js="true"></ol>
-<button id="chat-messages-unlock">Chat scroll paused. Click to resume.</button>`;
+<button id="chat-messages-unlock" data-string="chat_scroll_paused">Chat scroll paused. Click to resume.</button>`;
 const jsmarkup_chat_users = `\
 <article id="chat-users_js">
   <h5 id="chat-users_js__watching-header"></h5>
@@ -37,31 +37,31 @@ const jsmarkup_chat_form = `\
   <div id="chat-live">
     <span id="chat-live__ball"></span>
     <span id="chat-live__status">
-      <span data-verbose="true">Not connected to chat</span>
+      <span data-verbose="true" data-string="not_connected_to_chat">Not connected to chat</span>
       <span data-verbose="false">&times;</span>
     </span>
   </div>
-  <input id="chat-form_js__submit" type="submit" value="Chat" accesskey="p" disabled>
+  <input id="chat-form_js__submit" type="submit" value="Chat" accesskey="p" disabled data-string="chat" data-string-attr="value">
   <input id="chat-form_js__captcha-digest" type="hidden" name="captcha-digest" disabled>
   <input id="chat-form_js__captcha-image" type="image" width="72" height="30">
-  <input id="chat-form_js__captcha-answer" name="captcha-answer" placeholder="Captcha" disabled>
-  <input id="chat-form_js__settings" type="image" src="/static/settings.svg" width="28" height="28" alt="Settings">
+  <input id="chat-form_js__captcha-answer" name="captcha-answer" placeholder="Captcha" disabled data-string="captcha" data-string-attr="placeholder">
+  <input id="chat-form_js__settings" type="image" src="/static/settings.svg" width="28" height="28" alt="Settings" data-string="settings" data-string-attr="alt">
   <article id="chat-form_js__notice">
     <button id="chat-form_js__notice__button" type="button">
       <header id="chat-form_js__notice__button__header"></header>
-      <small>Click to dismiss</small>
+      <small data-string="click_to_dismiss">Click to dismiss</small>
     </button>
   </article>
 </form>
 <form id="appearance-form_js" data-hidden="">
-  <span id="appearance-form_js__label-name">Name:</span>
+  <span id="appearance-form_js__label-name" data-string="name">Name:</span>
   <input id="appearance-form_js__name" name="name">
   <input id="appearance-form_js__color" type="color" name="color">
-  <span id="appearance-form_js__label-tripcode">Tripcode:</span>
-  <input id="appearance-form_js__password" type="password" name="password" placeholder="(tripcode password)">
+  <span id="appearance-form_js__label-tripcode" data-string="tripcode">Tripcode:</span>
+  <input id="appearance-form_js__password" type="password" name="password" placeholder="(tripcode password)" data-string="tripcode_password" data-string-attr="placeholder">
   <div id="appearance-form_js__row">
     <article id="appearance-form_js__row__result"></article>
-    <input id="appearance-form_js__row__submit" type="submit" value="Update">
+    <input id="appearance-form_js__row__submit" type="submit" value="Update" data-string="update" data-string-attr="value">
   </div>
 </form>`;
 
@@ -247,7 +247,7 @@ const create_chat_user_components = (user) => {
   } else {
     const chat_user_insignia = document.createElement("b");
     chat_user_insignia.classList.add("chat-insignia")
-    chat_user_insignia.title = "Broadcaster";
+    chat_user_insignia.title = locale.broadcaster || "Broadcaster";
     chat_user_insignia.innerText = "##";
     const chat_user_insignia_nbsp = document.createElement("span");
     chat_user_insignia_nbsp.innerHTML = "&nbsp;"
@@ -275,6 +275,7 @@ const delete_chat_messages = (seqs) => {
   }
 }
 
+let locale = {};
 let users = {};
 let stats = null;
 let stats_received = null;
@@ -438,23 +439,23 @@ const chat_form_captcha_answer = document.getElementById("chat-form_js__captcha-
 chat_form_captcha_image.addEventListener("loadstart", (event) => {
   chat_form_captcha_image.removeAttribute("title");
   chat_form_captcha_image.removeAttribute("data-reloadable");
-  chat_form_captcha_image.alt = "Loading...";
+  chat_form_captcha_image.alt = locale.loading || "Loading...";
 });
 chat_form_captcha_image.addEventListener("load", (event) => {
   chat_form_captcha_image.removeAttribute("alt");
   chat_form_captcha_image.dataset.reloadable = "";
-  chat_form_captcha_image.title = "Click for a new captcha";
+  chat_form_captcha_image.title = locale.click_for_a_new_captcha || "Click for a new captcha";
 });
 chat_form_captcha_image.addEventListener("error", (event) => {
-  chat_form_captcha_image.alt = "Captcha failed to load";
+  chat_form_captcha_image.alt = locale.captcha_failed_to_load || "Captcha failed to load";
   chat_form_captcha_image.dataset.reloadable = "";
-  chat_form_captcha_image.title = "Click for a new captcha";
+  chat_form_captcha_image.title = locale.click_for_a_new_captcha || "Click for a new captcha";
 });
 chat_form_captcha_image.addEventListener("click", (event) => {
   event.preventDefault();
   if (chat_form_captcha_image.dataset.reloadable !== undefined) {
     chat_form_submit.disabled = true;
-    chat_form_captcha_image.alt = "Waiting...";
+    chat_form_captcha_image.alt = locale.waiting || "Waiting...";
     chat_form_captcha_image.removeAttribute("title");
     chat_form_captcha_image.removeAttribute("data-reloadable");
     chat_form_captcha_image.removeAttribute("src");
@@ -518,7 +519,7 @@ const update_uptime = () => {
 setInterval(update_uptime, 1000); // always update uptime
 
 const update_viewership = () => {
-  info_viewership.innerText = stats === null ? "" : `${stats.viewership} viewers`;
+  info_viewership.innerText = stats === null ? "" : (locale.viewers || "{0} viewers").replace('{0}', stats.viewership);
 }
 
 const update_stats = () => {
@@ -564,7 +565,7 @@ const update_users_list = () => {
     }
     if (is_you) {
       const you = document.createElement("span");
-      you.innerText = " (You)";
+      you.innerText = locale.you || " (You)";
       chat_user.insertAdjacentElement("beforeend", you);
     }
     chat_users_sublist.insertAdjacentElement("beforeend", chat_user);
@@ -584,8 +585,8 @@ const update_users_list = () => {
   }
 
   // show correct numbers
-  chat_users_watching_header.innerText = `Watching (${watching})`;
-  chat_users_notwatching_header.innerText = `Not watching (${notwatching})`;
+  chat_users_watching_header.innerText = (locale.watching || "Watching ({0})").replace("{0}", watching);
+  chat_users_notwatching_header.innerText = (locale.not_watching || "Not watching ({0})").replace("{0}", notwatching);
 }
 
 const show_offline_screen = () => {
@@ -608,6 +609,21 @@ const on_websocket_message = async (event) => {
       console.log("ws init", receipt);
 
       pingpong_period = receipt.pingpong;
+
+      // update locale & put localized strings in js-inserted elements
+      locale = receipt.locale;
+      for (element of document.querySelectorAll('[data-string]')) {
+        const string = element.dataset.string;
+        if (locale[string] !== undefined) {
+          const attr = element.dataset.stringAttr;
+          if (attr === undefined)
+            element.innerText = locale[string];
+          else
+            element[attr] = locale[string];
+        }
+      }
+
+      // stream title
       set_title(receipt.title);
 
       // update stats (uptime/viewership)
@@ -813,7 +829,7 @@ const on_websocket_message = async (event) => {
           ul.insertAdjacentElement("beforeend", li);
         }
         const result = document.createElement("div");
-        result.innerText = "Errors:";
+        result.innerText = locale.errors || "Errors:";
         result.insertAdjacentElement("beforeend", ul);
         chat_appearance_form_result.innerHTML = result.innerHTML;
       }
@@ -850,14 +866,14 @@ const connect_websocket = () => {
     return;
   }
   chat_live_ball.style.borderColor = "gold";
-  chat_live_status.innerHTML = "<span data-verbose='true'>Connecting to chat...</span><span data-verbose='false'>&middot;&middot;&middot;</span>";
+  chat_live_status.innerHTML = `<span data-verbose='true'>${locale.connecting_to_chat || "Connecting to chat..."}</span><span data-verbose='false'>&middot;&middot;&middot;</span>`;
   ws = null;
   ws = new WebSocket(`ws://${document.domain}:${location.port}/live?token=${encodeURIComponent(TOKEN)}`);
   ws.addEventListener("open", (event) => {
     console.log("websocket open", event);
     chat_form_submit.disabled = false;
     chat_live_ball.style.borderColor = "green";
-    chat_live_status.innerHTML = "<span><span data-verbose='true'>Connected to chat</span><span data-verbose='false'>&check;</span></span>";
+    chat_live_status.innerHTML = `<span><span data-verbose='true'>${locale.connected_to_chat || "Connected to chat"}</span><span data-verbose='false'>&check;</span></span>`;
     // When the server is offline, a newly opened websocket can take a second
     // to close. This timeout tries to ensure the backoff doesn't instantly
     // (erroneously) reset to 2 seconds in that case.
@@ -873,7 +889,7 @@ const connect_websocket = () => {
     console.log("websocket close", event);
     chat_form_submit.disabled = true;
     chat_live_ball.style.borderColor = "maroon";
-    chat_live_status.innerHTML = "<span data-verbose='true'>Disconnected from chat</span><span data-verbose='false'>&times;</span>";
+    chat_live_status.innerHTML = `<span data-verbose='true'>${locale.disconnected_from_chat || "Disconnected from chat"}</span><span data-verbose='false'>&times;</span>`;
     if (!ws.successor) {
       ws.successor = true;
       setTimeout(connect_websocket, websocket_backoff);
@@ -884,7 +900,7 @@ const connect_websocket = () => {
     console.log("websocket error", event);
     chat_form_submit.disabled = true;
     chat_live_ball.style.borderColor = "maroon";
-    chat_live_status.innerHTML = "<span>Error<span data-verbose='true'> connecting to chat</span></span>";
+    chat_live_status.innerHTML = `<span><span data-verbose='true'>${locale.error_connecting_to_chat || "Error connecting to chat"}</span><span data-verbose='false'>${locale.error_connecting_to_chat_terse || "Error"}</span></span>`;
   });
   ws.addEventListener("message", on_websocket_message);
 }
diff --git a/anonstream/templates/captcha.html b/anonstream/templates/captcha.html
index 3f21ee3..780601f 100644
--- a/anonstream/templates/captcha.html
+++ b/anonstream/templates/captcha.html
@@ -57,7 +57,7 @@
   <img src="{{ url_for('captcha', digest=digest) }}" width="72" height="30">
   <form action="{{ url_for('access', token=token) }}" method="post">
     <input type="hidden" name="digest" value="{{ digest }}">
-    <input name="answer" placeholder="Captcha" required autofocus>
+    <input name="answer" placeholder="{{ locale.captcha }}" required autofocus>
     <input type="submit" value="Submit">
     {% if failure is not none %}<p>{{ failure }}</p>{% endif %}
   </form>
diff --git a/anonstream/templates/error.html b/anonstream/templates/error.html
index ab8d9cc..404c99d 100644
--- a/anonstream/templates/error.html
+++ b/anonstream/templates/error.html
@@ -7,7 +7,7 @@
 <head>
   <meta charset="utf-8">
   <meta name="viewport" content="width=device-width, initial-scale=1.0">
-  <title>{{ error.code }} {{ error.name }}</title>
+  <title>{{ error.code }} {{ locale[error.code | string] or error.name }}</title>
   <style>
     body {
       background-color: #232327;
@@ -63,8 +63,8 @@
 </head>
 <body>
   <main>
-    <h1>{{ error.name }}</h1>
-    {% if error.description != error.__class__.description %}
+    <h1>{{ locale[error.code | string] or error.name }}</h1>
+    {% if error.description is not none %}
       <p>{{ error.description }}</p>
     {% endif %}
   </main>
diff --git a/anonstream/templates/home.html b/anonstream/templates/home.html
index 696c435..5454a3f 100644
--- a/anonstream/templates/home.html
+++ b/anonstream/templates/home.html
@@ -3,12 +3,12 @@
  # SPDX-License-Identifier: AGPL-3.0-or-later
  ##}
 <!doctype html>
-<html id="nochat">
+<html id="nochat" lang="{{ lang }}">
 <head>
   <meta charset="utf-8">
   <meta name="viewport" content="width=device-width, initial-scale=1.0">
   <meta http-equiv="content-security-policy" content="default-src 'none'; connect-src 'self'; img-src 'self'; frame-src 'self'; media-src 'self'; script-src 'self'; style-src 'self' 'nonce-{{ csp }}';">
-  <link rel="stylesheet" href="{{ url_for('static', filename='style.css') }}" type="text/css">
+  <link rel="stylesheet" href="{{ url_for('static', filename='style.css', token=user.token) }}" type="text/css">
 </head>
 <body id="both" data-token="{{ user.token }}" data-token-hash="{{ user.token_hash }}" data-csp="{{ csp }}">
   <article id="stream">
@@ -20,15 +20,15 @@
   <aside id="chat">
     <input id="chat__toggle" type="checkbox">
     <header id="chat__header">
-      <label id="chat__header__button" for="chat__toggle">Users</label>
-      <h3 id="chat__header__text">Stream chat</h3>
+      <label id="chat__header__button" for="chat__toggle">{{ locale.users }}</label>
+      <h3 id="chat__header__text">{{ locale.stream_chat }}</h3>
     </header>
     <article id="chat__body">
       <section id="chat__body__messages">
         <noscript><iframe id="chat-messages_nojs" src="{{ url_for('nojs_chat_messages', token=user.token, _anchor='end') }}" data-js="false"></iframe></noscript>
       </section>
       <section id="chat__body__users">
-        <header id="chat-users-header"><h4>Users in chat</h4></header>
+        <header id="chat-users-header"><h4>{{ locale.users_in_chat }}</h4></header>
         <noscript><iframe id="chat-users_nojs" src="{{ url_for('nojs_chat_users', token=user.token) }}" data-js="false"></iframe></noscript>
       </section>
     </article>
@@ -41,7 +41,7 @@
     <a href="#chat">chat</a>
     <a href="#both">both</a>
   </nav>
-  <footer>anonstream {{ version }} &mdash; <a href="https://git.076.ne.jp/ninya9k/anonstream" target="_blank">source</a></footer>
-  <script src="{{ url_for('static', filename='anonstream.js') }}" type="text/javascript"></script>
+  <footer>anonstream {{ version }} &mdash; <a href="https://git.076.ne.jp/ninya9k/anonstream" target="_blank">{{ locale.source }}</a></footer>
+  <script src="{{ url_for('static', filename='anonstream.js', token=user.token) }}" type="text/javascript"></script>
 </body>
 </html>
diff --git a/anonstream/templates/nojs_chat_form.html b/anonstream/templates/nojs_chat_form.html
index 8eeb650..afb3449 100644
--- a/anonstream/templates/nojs_chat_form.html
+++ b/anonstream/templates/nojs_chat_form.html
@@ -214,45 +214,50 @@
   <input id="toggle" type="checkbox" {% if not prefer_chat_form %}checked {% endif %}accesskey="x">
   {% if state.notice %}
     <input id="notice-radio" type="radio" accesskey="z">
-    <label id="notice" for="notice-radio"{% if state.verbose %} class="verbose"{% endif %}>
-      <header><h2>{{ state.notice }}</h2></header>
-      <small>Click to dismiss</small>
+    <label id="notice" for="notice-radio"{% if (state.notice | length) > 1 %} class="verbose"{% endif %}>
+      <header><h2>
+        {%- for string, args in state.notice %}
+          {{ locale.internal[string] | escape | format(*args) -}}
+          {% if not loop.last %}<br>{% endif %}
+        {% endfor -%}
+      </h2></header>
+      <small>{{ locale.form.click_to_dismiss }}</small>
     </label>
   {% endif %}
   <form id="chat-form" action="{{ url_for('nojs_submit_message', token=user.token) }}" method="post">
     <input type="hidden" name="nonce" value="{{ nonce }}">
-    <textarea id="chat-form__comment" name="comment" maxlength="{{ max_comment_length }}" {% if digest is none %}required {% endif %} placeholder="Send a message..." rows="1" tabindex="1" autofocus accesskey="m">{{ state.comment }}</textarea>
-    <input id="chat-form__submit" type="submit" value="Chat" tabindex="4" accesskey="p">
-    <div id="chat-form__exit"><label for="toggle" class="pseudolink">Settings</label></div>
+    <textarea id="chat-form__comment" name="comment" maxlength="{{ max_comment_length }}" {% if digest is none %}required {% endif %} placeholder="{{ locale.form.send_a_message }}" rows="1" tabindex="1" autofocus accesskey="m">{{ state.comment }}</textarea>
+    <input id="chat-form__submit" type="submit" value="{{ locale.form.chat }}" tabindex="4" accesskey="p">
+    <div id="chat-form__exit"><label for="toggle" class="pseudolink">{{ locale.form.settings }}</label></div>
     {% if digest %}
       <input type="hidden" name="captcha-digest" value="{{ digest }}">
-      <input id="chat-form__captcha-image" type="image" formaction="{{ url_for('nojs_chat_form_redirect', token=user.token) }}" formnovalidate src="{{ url_for('captcha', token=user.token, digest=digest) }}" width="72" height="30" alt="Captcha failed to load" title="Click for a new captcha" tabindex="2">
+      <input id="chat-form__captcha-image" type="image" formaction="{{ url_for('nojs_chat_form_redirect', token=user.token) }}" formnovalidate src="{{ url_for('captcha', token=user.token, digest=digest) }}" width="72" height="30" alt="{{ locale.form.captcha_failed_to_load }}" title="{{ locale.form.click_for_a_new_captcha }}" tabindex="2">
       <input id="chat-form__captcha-answer" name="captcha-answer" required placeholder="Captcha" tabindex="3">
     {% endif %}
   </form>
   <form id="appearance-form" action="{{ url_for('nojs_submit_appearance', token=user.token) }}" method="post">
-    <label id="appearance-form__label-name" for="appearance-form__name">Name:</label>
+    <label id="appearance-form__label-name" for="appearance-form__name">{{ locale.form.name }}</label>
     <input id="appearance-form__name" name="name" value="{{ user.name or '' }}" placeholder="{{ default_name }}" maxlength="{{ max_name_length }}">
     <input type="color" name="color" value="{{ user.color }}">
-    <label id="appearance-form__label-password" for="appearance-form__password">Tripcode:</label>
+    <label id="appearance-form__label-password" for="appearance-form__password">{{ locale.form.tripcode }}</label>
     <input id="password-toggle" name="set-tripcode" type="checkbox" accesskey="s">
     <input id="cleared-toggle" name="clear-tripcode" type="checkbox"{% if user.tripcode != none %} accesskey="c"{% endif %}>
     <div id="password-column">
       {% if user.tripcode is none %}
-        <span class="tripcode">(no tripcode)</span>
-        <label for="password-toggle" class="show-password pseudolink">set</label>
+        <span class="tripcode">{{ locale.form.no_tripcode }}</span>
+        <label for="password-toggle" class="show-password pseudolink">{{ locale.form.set }}</label>
       {% else %}
         <label id="tripcode" for="password-toggle" class="show-password tripcode">{{ user.tripcode.digest }}</label>
         <label id="show-cleared" for="cleared-toggle" class="pseudolink x">&times;</label>
-        <div id="cleared" class="tripcode">(cleared)</div>
-        <label id="hide-cleared" for="cleared-toggle" class="pseudolink">undo</label>
+        <div id="cleared" class="tripcode">{{ locale.form.cleared }}</div>
+        <label id="hide-cleared" for="cleared-toggle" class="pseudolink">{{ locale.form.undo }}</label>
       {% endif %}
     </div>
-    <input id="appearance-form__password" name="password" type="password" placeholder="(tripcode password)" maxlength="{{ max_password_length }}">
+    <input id="appearance-form__password" name="password" type="password" placeholder="{{ locale.form.tripcode_password }}" maxlength="{{ max_password_length }}">
     <div id="hide-password"><label for="password-toggle" class="pseudolink x">&times;</label></div>
     <div id="appearance-form__buttons">
-      <div id="appearance-form__buttons__exit"><label for="toggle" class="pseudolink">Return to chat</label></div>
-      <input type="submit" value="Update">
+      <div id="appearance-form__buttons__exit"><label for="toggle" class="pseudolink">{{ locale.form.return_to_chat }}</label></div>
+      <input type="submit" value="{{ locale.form['update'] }}">
     </div>
   </form>
 </body>
diff --git a/anonstream/templates/nojs_chat_messages.html b/anonstream/templates/nojs_chat_messages.html
index 91999cd..ccdbd01 100644
--- a/anonstream/templates/nojs_chat_messages.html
+++ b/anonstream/templates/nojs_chat_messages.html
@@ -165,8 +165,8 @@
   <div id="notimeout"></div>
   <aside id="timeout">
     <a class="button" href="{{ url_for('nojs_chat_messages_redirect', token=user.token) }}">
-      <header>Timed out</header>
-      <small>Click to refresh</small>
+      <header>{{ locale.timed_out }}</header>
+      <small>{{ locale.click_to_refresh }}</small>
     </a>
   </aside>
   <ol id="chat-messages">
@@ -183,10 +183,10 @@
     {% endfor %}
   </ol>
   <aside id="timeout-dismiss">
-    <a class="button" href="#notimeout">Hide timeout notice</a>
+    <a class="button" href="#notimeout">{{ locale.hide_timeout_notice }}</a>
   </aside>
   <aside id="timeout-alt">
-    <a class="button" href="{{ url_for('nojs_chat_messages_redirect', token=user.token) }}">Click to refresh</a>
+    <a class="button" href="{{ url_for('nojs_chat_messages_redirect', token=user.token) }}">{{ locale.click_to_refresh }}</a>
   </aside>
 </body>
 </html>
diff --git a/anonstream/templates/nojs_chat_users.html b/anonstream/templates/nojs_chat_users.html
index a5da3b0..131dddd 100644
--- a/anonstream/templates/nojs_chat_users.html
+++ b/anonstream/templates/nojs_chat_users.html
@@ -109,27 +109,27 @@
 <body>
   <aside id="timeout">
     <a href="">
-      <header>Timed out</header>
-      <small>Click to refresh</small>
+      <header>{{ locale.timed_out}} </header>
+      <small>{{ locale.click_to_refresh }}</small>
     </a>
   </aside>
   <main id="main">
-    <h5>Watching ({{ users_watching | length }})</h5>
+    <h5>{{ locale.watching | format(users_watching | length) }}</h5>
     <ul>
       {% for user_listed in users_watching %}
         <li class="user" data-token-hash="{{ user_listed.token_hash }}">
           {{- appearance(user_listed, insignia_class='user__insignia', name_class='user__name', tag_class='user__name__tag') -}}
-          {%- if user.token == user_listed.token %} (You){% endif -%}
+          {%- if user.token == user_listed.token %}{{ locale.you }}{% endif -%}
         </li>
       {% endfor %}
     </ul>
     <br>
-    <h5>Not watching ({{ users_notwatching | length }})</h5>
+    <h5>{{ locale.not_watching | format(users_notwatching | length) }}</h5>
     <ul>
       {% for user_listed in users_notwatching %}
         <li class="user" data-token-hash="{{ user_listed.token_hash }}">
           {{- appearance(user_listed, insignia_class='user__insignia', name_class='user__name', tag_class='user__name__tag') -}}
-          {%- if user.token == user_listed.token %} (You){% endif -%}
+          {%- if user.token == user_listed.token %}{{ locale.you }}{% endif -%}
         </li>
       {% endfor %}
     </ul>
diff --git a/anonstream/templates/nojs_info.html b/anonstream/templates/nojs_info.html
index 362abb1..b5583bd 100644
--- a/anonstream/templates/nojs_info.html
+++ b/anonstream/templates/nojs_info.html
@@ -144,13 +144,13 @@
       {% if user.presence != Presence.WATCHING %}
         <form id="float__form" action="{{ url_for('nojs_stream') }}" target="stream_nojs">
           <input type="hidden" name="token" value="{{ user.token }}">
-          <input type="submit" value="Reload stream" accesskey="r">
+          <input type="submit" value="{{ locale.reload_stream }}" accesskey="r">
         </form>
       {% endif %}
-      <div id="float__viewership">{{ viewership }} viewers</div>
+      <div id="float__viewership">{{ locale.viewers | format(viewership) }}</div>
       <div id="float__uptime">
         <div id="uptime-static"{% if uptime < 360000 %} data-hidden=""{% endif %}>
-          <span id="uptime-static__label">Uptime:</span>
+          <span id="uptime-static__label">{{ locale.uptime }}</span>
           <span>
             {%- if uptime >= 3600 -%}
               {{- uptime | int // 3600 -}}
diff --git a/anonstream/templates/nojs_stream.html b/anonstream/templates/nojs_stream.html
index 5eba22a..da0afb5 100644
--- a/anonstream/templates/nojs_stream.html
+++ b/anonstream/templates/nojs_stream.html
@@ -45,7 +45,7 @@
   {% if online %}
     <video id="video" src="{{ url_for('stream', token=user.token) }}" autoplay controls></video>
   {% else %}
-    <header id="offline"><h1>[offline]</h1></header>
+    <header id="offline"><h1>{{ locale.offline }}</h1></header>
   {% endif %}
 </body>
 </html>
diff --git a/anonstream/user.py b/anonstream/user.py
index 5ecc711..60f9107 100644
--- a/anonstream/user.py
+++ b/anonstream/user.py
@@ -6,7 +6,7 @@ import time
 from functools import reduce
 from math import inf
 
-from quart import current_app
+from quart import current_app, Markup
 
 from anonstream.wrappers import try_except_log, with_timestamp, get_timestamp
 from anonstream.helpers.user import get_default_name, get_presence, Presence
@@ -106,10 +106,11 @@ def change_name(user, name, dry_run=False):
             name = None
         if name is not None:
             if len(name) == 0:
-                raise BadAppearance('Name was empty')
+                raise BadAppearance('name_empty')
             if len(name) > CONFIG['CHAT_NAME_MAX_LENGTH']:
                 raise BadAppearance(
-                    f'Name exceeded {CONFIG["CHAT_NAME_MAX_LENGTH"]} chars'
+                    'name_too_long',
+                    CONFIG['CHAT_NAME_MAX_LENGTH'],
                 )
     else:
         user['name'] = name
@@ -119,16 +120,13 @@ def change_color(user, color, dry_run=False):
         try:
             colour = color_to_colour(color)
         except NotAColor:
-            raise BadAppearance('Invalid CSS color')
-        contrast = get_contrast(
-            CONFIG['CHAT_BACKGROUND_COLOUR'],
-            colour,
-        )
+            raise BadAppearance('colour_invalid_css')
+        contrast = get_contrast(CONFIG['CHAT_BACKGROUND_COLOUR'], colour)
         min_contrast = CONFIG['CHAT_NAME_MIN_CONTRAST']
         if contrast < min_contrast:
             raise BadAppearance(
-                'Colour had insufficient contrast:',
-                (f'{contrast:.2f}', f'/{min_contrast:.2f}'),
+                'colour_insufficient_contrast',
+                Markup(f'<mark>{contrast:.2f}</mark>/{min_contrast:.2f}'),
             )
     else:
         user['color'] = color
@@ -137,8 +135,8 @@ def change_tripcode(user, password, dry_run=False):
     if dry_run:
         if len(password) > CONFIG['CHAT_TRIPCODE_PASSWORD_MAX_LENGTH']:
             raise BadAppearance(
-                f'Password exceeded '
-                f'{CONFIG["CHAT_TRIPCODE_PASSWORD_MAX_LENGTH"]} chars'
+                'password_too_long',
+                CONFIG['CHAT_TRIPCODE_PASSWORD_MAX_LENGTH'],
             )
     else:
         user['tripcode'] = generate_tripcode(password)
@@ -176,11 +174,11 @@ def verify(user, digest, answer):
     else:
         match check_captcha_digest(CAPTCHA_SIGNER, digest, answer):
             case Answer.MISSING:
-                raise BadCaptcha('Captcha is required')
+                raise BadCaptcha('captcha_required')
             case Answer.BAD:
-                raise BadCaptcha('Captcha was incorrect')
+                raise BadCaptcha('captcha_incorrect')
             case Answer.EXPIRED:
-                raise BadCaptcha('Captcha has expired')
+                raise BadCaptcha('captcha_expired')
             case Answer.OK:
                 user['verified'] = True
                 verification_happened = True
diff --git a/anonstream/utils/locale.py b/anonstream/utils/locale.py
new file mode 100644
index 0000000..c0e8edd
--- /dev/null
+++ b/anonstream/utils/locale.py
@@ -0,0 +1,135 @@
+import types
+
+SPEC = {
+    'anonstream': {
+        'error': {
+            'blacklisted': str,
+            'not_whitelisted': str,
+            'offline': str,
+            'ratelimit': str,
+            'limit': str,
+        },
+        'internal': {
+            'captcha_required': str,
+            'captcha_incorrect': str,
+            'captcha_expired': str,
+            'message_ratelimited': str,
+            'message_suspected_duplicate': str,
+            'message_empty': str,
+            'message_practically_empty': str,
+            'message_too_long': str,
+            'message_too_many_lines': str,
+            'message_too_many_apparent_lines': str,
+            'appearance_changed': str,
+            'name_empty': str,
+            'name_too_long': str,
+            'colour_invalid_css': str,
+            'colour_insufficient_contrast': str,
+            'password_too_long': str,
+        },
+        'captcha': {
+            'captcha_failed_to_load': str,
+            'click_for_a_new_captcha': str,
+        },
+        'home': {
+            'source': str,
+            'users': str,
+            'users_in_chat': str,
+            'stream_chat': str,
+        },
+        'stream': {
+            'offline': str,
+        },
+        'info': {
+            'viewers': str,
+            'uptime': str,
+            'reload_stream': str,
+        },
+        'chat': {
+            'users': str,
+            'click_to_refresh': str,
+            'hide_timeout_notice': str,
+            'watching': str,
+            'not_watching': str,
+            'you': str,
+            'timed_out': str,
+        },
+        'form': {
+            'click_to_dismiss': str,
+            'send_a_message': str,
+            'captcha': str,
+            'settings': str,
+            'captcha_failed_to_load': str,
+            'click_for_a_new_captcha': str,
+            'chat': str,
+            'name': str,
+            'tripcode': str,
+            'no_tripcode': str,
+            'set': str,
+            'cleared': str,
+            'undo': str,
+            'tripcode_password': str,
+            'return_to_chat': str,
+            'update': str,
+        },
+        'js': {
+            'offline': str,
+            'reload_stream': str,
+            'chat_scroll_paused': str,
+            'not_connected': str,
+            'broadcaster': str,
+            'loading': str,
+            'click_for_a_new_captcha': str,
+            'viewers': str,
+            'you': str,
+            'watching': str,
+            'not_watching': str,
+            'errors': str,
+            'connecting_to_chat': str,
+            'connected_to_chat': str,
+            'disconnected_from_chat': str,
+            'error_connecting_to_chat': str,
+            'error_connecting_to_chat_terse': str,
+        }
+    },
+    'http': {
+        '400': str | None,
+        '401': str | None,
+        '403': str | None,
+        '404': str | None,
+        '405': str | None,
+        '410': str | None,
+        '500': str | None,
+    }
+}
+
+class Nonconforming(Exception):
+    pass
+
+def _conform_to_spec(data, spec, level=()):
+    assert isinstance(spec, dict), \
+        f'bad locale spec at {level}: must be {dict}, not {type(spec)}'
+    if not isinstance(data, dict):
+        raise Nonconforming(
+            f'object at {level} must be dict, not {type(data)}'
+        )
+    missing_keys = set(spec.keys()) - set(data.keys())
+    if missing_keys:
+        raise Nonconforming(f'dict at {level} is missing keys {missing_keys}')
+    extra_keys = set(data.keys()) - set(spec.keys())
+    if extra_keys:
+        raise Nonconforming(f'dict at {level} has extra keys {extra_keys}')
+    for key, subspec in spec.items():
+        subdata = data[key]
+        if isinstance(subspec, dict):
+            _conform_to_spec(subdata, subspec, level + (key,))
+        else:
+            assert isinstance(subspec, type | types.UnionType), \
+                f'bad locale spec at {level + (key,)}: must be {dict | type}, not {type(subspec)}'
+            if not isinstance(subdata, subspec):
+                raise Nonconforming(
+                    f'value at {level + (key,)} must be {subspec}, not {type(subdata)}'
+                )
+
+def validate_locale(locale):
+    return _conform_to_spec(locale, SPEC)
diff --git a/anonstream/utils/user.py b/anonstream/utils/user.py
index 98120b2..9d2601c 100644
--- a/anonstream/utils/user.py
+++ b/anonstream/utils/user.py
@@ -25,17 +25,6 @@ Presence = Enum(
 def generate_token():
     return secrets.token_hex(16)
 
-def concatenate_for_notice(string, *tuples):
-    if not tuples:
-        return string
-    markup = Markup(
-        ''.join(
-            f' <mark>{escape(x)}</mark>{escape(y)}'
-            for x, y in tuples
-        )
-    )
-    return string + markup
-
 def trilean(presence):
     match presence:
         case Presence.WATCHING:
diff --git a/anonstream/websocket.py b/anonstream/websocket.py
index 48d8467..dea3feb 100644
--- a/anonstream/websocket.py
+++ b/anonstream/websocket.py
@@ -9,6 +9,7 @@ from quart import current_app, websocket
 from anonstream.stream import get_stream_title, get_stream_uptime_and_viewership
 from anonstream.captcha import get_random_captcha_digest_for
 from anonstream.chat import get_all_messages_for_websocket, add_chat_message, Rejected
+from anonstream.locale import get_locale_from
 from anonstream.user import get_all_users_for_websocket, see, reading, verify, deverify, BadCaptcha, try_change_appearance, ensure_allowedness, AllowednessException
 from anonstream.wrappers import with_timestamp, get_timestamp
 from anonstream.utils.chat import generate_nonce
@@ -36,6 +37,7 @@ async def websocket_outbound(queue, user):
         'scrollback': CONFIG['MAX_CHAT_SCROLLBACK'],
         'digest': get_random_captcha_digest_for(user),
         'pingpong': CONFIG['TASK_BROADCAST_PING'],
+        'locale': get_locale_from(websocket)['anonstream']['js'],
     })
     while True:
         payload = await queue.get()
@@ -126,7 +128,7 @@ def handle_inbound_appearance(timestamp, queue, user, name, color, password, wan
     else:
         return {
             'type': 'appearance',
-            'result': 'Changed appearance',
+            'result': 'Changed appearance' " [THIS STRING STILL HARDCODED]",
             'name': user['name'],
             'color': user['color'],
             #'tripcode': user['tripcode'],
diff --git a/config.toml b/config.toml
index 1a205d7..f0afa0e 100644
--- a/config.toml
+++ b/config.toml
@@ -1,5 +1,10 @@
 secret_key = "place secret key here"
 
+[locale]
+default = "en"
+offered = ["en"]
+directory = "l10n/"
+
 [socket.control]
 enabled = true
 address = "control.sock"
diff --git a/l10n/en.json b/l10n/en.json
new file mode 100644
index 0000000..ec71a22
--- /dev/null
+++ b/l10n/en.json
@@ -0,0 +1,102 @@
+{
+  "anonstream": {
+    "error": {
+      "blacklisted": "You have been blacklisted.",
+      "not_whitelisted": "You have not been whitelisted.",
+      "offline": "The stream is offline.",
+      "ratelimit": "You have requested the stream recently. Try again in %.1f seconds.",
+      "limit": "You have made %d concurrent requests or the stream. End one of those before making a new request."
+    },
+    "internal": {
+      "captcha_required": "Captcha required",
+      "captcha_incorrect": "Captcha incorrect",
+      "captcha_expired": "Captcha expired",
+      "message_ratelimited": "Chat overuse in the last %.0f seconds",
+      "message_suspected_duplicate": "Discarded suspected duplicate message",
+      "message_empty": "Message was empty",
+      "message_practically_empty": "Message was practically empty",
+      "message_too_long": "Message exceeded %d chars",
+      "message_too_many_lines": "Message exceeded %d lines",
+      "message_too_many_apparent_lines": "Message would span %d or more lines",
+      "appearance_changed": "Changed appearance",
+      "name_empty": "Name was empty",
+      "name_too_long": "Name exceeded %d chars",
+      "colour_invalid_css": "Invalid CSS color",
+      "colour_insufficient_contrast": "Colour had insufficient contrast: %s",
+      "password_too_long": "Password exceeded %d chars"
+    },
+    "captcha": {
+      "captcha_failed_to_load": "Captcha failed to load",
+      "click_for_a_new_captcha": "Click for a new captcha"
+    },
+    "home": {
+      "users": "Users",
+      "stream_chat": "Stream chat",
+      "users_in_chat": "Users in chat",
+      "source": "source"
+    },
+    "stream": {
+      "offline": "[offline]"
+    },
+    "info": {
+      "viewers": "%d viewers",
+      "uptime": "Uptime:",
+      "reload_stream": "Reload stream"
+    },
+    "chat": {
+      "users": "Users",
+      "click_to_refresh": "Click to refresh",
+      "hide_timeout_notice": "Hide timeout notice",
+      "watching": "Watching (%d)",
+      "not_watching": "Not watching (%d)",
+      "you": " (You)",
+      "timed_out": "Timed out"
+    },
+    "form": {
+      "click_to_dismiss": "Click to dismiss",
+      "send_a_message": "Send a message...",
+      "captcha": "Captcha",
+      "settings": "Settings",
+      "captcha_failed_to_load": "Captcha failed to load",
+      "click_for_a_new_captcha": "Click for a new captcha",
+      "chat": "Chat",
+      "name": "Name:",
+      "tripcode": "Tripcode:",
+      "no_tripcode": "(no tripcode)",
+      "set": "set",
+      "cleared": "(cleared)",
+      "undo": "undo",
+      "tripcode_password": "(tripcode password)",
+      "return_to_chat": "Return to chat",
+      "update": "Update"
+    },
+    "js": {
+      "offline": "[offline]",
+      "reload_stream": "Reload stream",
+      "chat_scroll_paused": "Chat scroll paused. Click to resume.",
+      "not_connected": "Not connected to chat",
+      "broadcaster": "Broadcaster",
+      "loading": "Loading...",
+      "click_for_a_new_captcha": "Click for a new captcha",
+      "viewers": "{0} viewers",
+      "you": " (You)",
+      "watching": "Watching ({0})",
+      "not_watching": "Not watching ({0})",
+      "errors": "Errors:",
+      "connecting_to_chat": "Connecting to chat...",
+      "connected_to_chat": "Connected to chat",
+      "disconnected_from_chat": "Disconnected from chat",
+      "error_connecting_to_chat": "Error connecting to chat",
+      "error_connecting_to_chat_terse": "Error"
+    }
+  },
+  "http": {
+    "400": null,
+    "401": null,
+    "403": null,
+    "404": null,
+    "405": null,
+    "410": null,
+    "500": null
+  }
+}
diff --git a/l10n/ru.json b/l10n/ru.json
new file mode 100644
index 0000000..671767e
--- /dev/null
+++ b/l10n/ru.json
@@ -0,0 +1,13 @@
+{
+  "anonstream": {
+  },
+  "http": {
+    "400": "Плохой запрос",
+    "401": "Неавторизованно",
+    "403": "Запрещено",
+    "404": "Не найден",
+    "405": "Метод не разрешён",
+    "410": null,
+    "500": "Внутренняя ошибка сервера"
+  }
+}