diff --git a/.cursor/rules/frontend.mdc b/.cursor/rules/frontend.mdc
index fe4a2b1..c4c209b 100644
--- a/.cursor/rules/frontend.mdc
+++ b/.cursor/rules/frontend.mdc
@@ -35,7 +35,7 @@ export const state = {
   current: new Date(),          // currently displayed month
   lastDutiesForList: [],        // duties array for the duty list
   todayRefreshInterval: null,   // interval handle
-  lang: "ru"                    // 'ru' | 'en'
+  lang: "en"                    // 'ru' | 'en'
 };
 ```
 
@@ -58,7 +58,7 @@ t(lang, key, params?)  // → translated string
 ```
 
 - `lang` is `'ru'` or `'en'`, stored in `state.lang`.
-- `getLang()` resolves language from: Telegram `initData` user → `navigator.language` → `"ru"`.
+- `getLang()` reads **backend config** only: `window.__DT_LANG` (set by `/app/config.js` from `DEFAULT_LANGUAGE`). If missing or invalid, falls back to `"en"`. No Telegram or navigator language detection.
 - Params use named placeholders: `t(lang, "duty.until", { time: "14:00" })`.
 - Fallback chain: `MESSAGES[lang][key]` → `MESSAGES.en[key]` → raw key string.
 - All user-visible text must go through `t()` — never hardcode Russian strings in JS.
diff --git a/.env.example b/.env.example
index 1403447..bb36433 100644
--- a/.env.example
+++ b/.env.example
@@ -26,7 +26,7 @@ ADMIN_USERNAMES=admin1,admin2
 # When the pinned duty message is updated on schedule, re-pin so members get a notification (default: 1). Set to 0 or false to disable.
 # DUTY_PIN_NOTIFY=1
 
-# Default UI language when user language is unknown: en or ru (default: en).
+# Single source of language for bot, API, and Mini App (en or ru). Default: en. No auto-detection.
 # DEFAULT_LANGUAGE=en
 
 # Reject Telegram initData older than this (seconds). 0 = do not check (default).
diff --git a/docs/configuration.md b/docs/configuration.md
index 130049d..43bd72b 100644
--- a/docs/configuration.md
+++ b/docs/configuration.md
@@ -19,7 +19,7 @@ All configuration is read from the environment (e.g. `.env` via python-dotenv).
 | **EXTERNAL_CALENDAR_ICS_URL** | string (URL) | *(empty)* | URL of a public ICS calendar (e.g. holidays). If set, those days are highlighted on the duty grid; users can tap "i" on a cell to see the event summary. Empty = no external calendar. |
 | **DUTY_DISPLAY_TZ** | string (timezone name) | `Europe/Moscow` | Timezone for the pinned duty message in groups. Example: `Europe/Moscow`, `UTC`. |
 | **DUTY_PIN_NOTIFY** | `0`, `false`, or `no` to disable | `1` (enabled) | When the pinned duty message is updated on schedule, the bot sends a new message, unpins the old one and pins the new one. If enabled, pinning the new message sends a Telegram notification (“Bot pinned a message”). Set to `0`, `false`, or `no` to pin without notification. The first pin (e.g. when the bot is added to the group or on `/pin_duty`) is always silent. |
-| **DEFAULT_LANGUAGE** | `en` or `ru` (normalized) | `en` | Default UI language when the user's Telegram language is unknown. Values starting with `ru` are normalized to `ru`, otherwise `en`. |
+| **DEFAULT_LANGUAGE** | `en` or `ru` (normalized) | `en` | **Single source of language for the whole deployment:** bot messages, API error texts, and Mini App UI all use this value. No auto-detection from Telegram user, browser, or `Accept-Language`. Values starting with `ru` are normalized to `ru`; anything else becomes `en`. |
 
 ## Roles and access
 
diff --git a/duty_teller/api/app.py b/duty_teller/api/app.py
index 1361288..21d17ed 100644
--- a/duty_teller/api/app.py
+++ b/duty_teller/api/app.py
@@ -5,7 +5,6 @@ import re
 from datetime import date, timedelta
 
 import duty_teller.config as config
-from starlette.middleware.base import BaseHTTPMiddleware
 
 from fastapi import Depends, FastAPI, Request
 from fastapi.middleware.cors import CORSMiddleware
@@ -58,22 +57,67 @@ app.add_middleware(
 )
 
 
-class NoCacheStaticMiddleware(BaseHTTPMiddleware):
-    """Set Cache-Control for /app/*.js and /app/*.html so WebView gets fresh JS (i18n, etc.)."""
+class NoCacheStaticMiddleware:
+    """
+    Raw ASGI middleware: Cache-Control: no-store for all /app and /app/* static files;
+    Vary: Accept-Language on all responses so reverse proxies do not serve one user's response to another.
+    """
 
-    async def dispatch(self, request, call_next):
-        response = await call_next(request)
-        path = request.url.path
-        if path.startswith("/app/") and (
-            path.endswith(".js") or path.endswith(".html")
-        ):
-            response.headers["Cache-Control"] = "no-store"
-        return response
+    def __init__(self, app, **kwargs):
+        self.app = app
+
+    async def __call__(self, scope, receive, send):
+        if scope["type"] != "http":
+            await self.app(scope, receive, send)
+            return
+
+        path = scope.get("path", "")
+        is_app_path = path == "/app" or path.startswith("/app/")
+
+        async def send_wrapper(message):
+            if message["type"] == "http.response.start":
+                headers = list(message.get("headers", []))
+                header_names = {h[0].lower(): i for i, h in enumerate(headers)}
+                if is_app_path:
+                    cache_control = (b"cache-control", b"no-store")
+                    if b"cache-control" in header_names:
+                        headers[header_names[b"cache-control"]] = cache_control
+                    else:
+                        headers.append(cache_control)
+                vary_val = b"Accept-Language"
+                if b"vary" in header_names:
+                    idx = header_names[b"vary"]
+                    existing = headers[idx][1]
+                    tokens = [p.strip() for p in existing.split(b",")]
+                    if vary_val not in tokens:
+                        headers[idx] = (b"vary", existing + b", " + vary_val)
+                else:
+                    headers.append((b"vary", vary_val))
+                message = {"type": "http.response.start", "status": message["status"], "headers": headers}
+            await send(message)
+
+        await self.app(scope, receive, send_wrapper)
 
 
 app.add_middleware(NoCacheStaticMiddleware)
 
 
+@app.get(
+    "/app/config.js",
+    summary="Mini App config (language)",
+    description="Returns JS that sets window.__DT_LANG from DEFAULT_LANGUAGE. Loaded before main.js.",
+)
+def app_config_js() -> Response:
+    """Return JS assigning window.__DT_LANG for the webapp. No caching."""
+    lang = config.DEFAULT_LANGUAGE
+    body = f'window.__DT_LANG = "{lang}";'
+    return Response(
+        content=body,
+        media_type="application/javascript; charset=utf-8",
+        headers={"Cache-Control": "no-store"},
+    )
+
+
 @app.get(
     "/api/duties",
     response_model=list[DutyWithUser],
diff --git a/duty_teller/api/dependencies.py b/duty_teller/api/dependencies.py
index 835adeb..f032598 100644
--- a/duty_teller/api/dependencies.py
+++ b/duty_teller/api/dependencies.py
@@ -1,7 +1,6 @@
 """FastAPI dependencies: DB session, Miniapp auth (initData/allowlist), date validation."""
 
 import logging
-import re
 from typing import Annotated, Generator
 
 from fastapi import Depends, Header, HTTPException, Query, Request
@@ -17,42 +16,17 @@ from duty_teller.db.repository import (
 from duty_teller.db.schemas import DUTY_EVENT_TYPES, DutyWithUser
 from duty_teller.db.session import session_scope
 from duty_teller.i18n import t
-from duty_teller.i18n.lang import normalize_lang
 from duty_teller.utils.dates import DateRangeValidationError, validate_date_range
 
 log = logging.getLogger(__name__)
 
-# Extract primary language code from first Accept-Language tag (e.g. "ru-RU" -> "ru").
-_ACCEPT_LANG_CODE_RE = re.compile(r"^([a-zA-Z]{2,3})(?:-|;|,|\s|$)")
-
-
-def _parse_first_language_code(header: str | None) -> str | None:
-    """Extract the first language code from Accept-Language header.
-
-    Args:
-        header: Raw Accept-Language value (e.g. "ru-RU,ru;q=0.9,en;q=0.8").
-
-    Returns:
-        Two- or three-letter code (e.g. 'ru', 'en') or None if missing/invalid.
-    """
-    if not header or not header.strip():
-        return None
-    first = header.strip().split(",")[0].strip()
-    m = _ACCEPT_LANG_CODE_RE.match(first)
-    return m.group(1).lower() if m else None
-
-
 def _lang_from_accept_language(header: str | None) -> str:
-    """Normalize Accept-Language header to 'ru' or 'en'; fallback to config.DEFAULT_LANGUAGE.
+    """Return the application language: always config.DEFAULT_LANGUAGE.
 
-    Args:
-        header: Raw Accept-Language header value (e.g. "ru-RU,ru;q=0.9,en;q=0.8").
-
-    Returns:
-        'ru' or 'en'.
+    The header argument is kept for backward compatibility but is ignored.
+    The whole deployment uses a single language from DEFAULT_LANGUAGE.
     """
-    code = _parse_first_language_code(header)
-    return normalize_lang(code if code is not None else config.DEFAULT_LANGUAGE)
+    return config.DEFAULT_LANGUAGE
 
 
 def _auth_error_detail(auth_reason: str, lang: str) -> str:
diff --git a/duty_teller/api/telegram_auth.py b/duty_teller/api/telegram_auth.py
index 73d7b9c..65d4128 100644
--- a/duty_teller/api/telegram_auth.py
+++ b/duty_teller/api/telegram_auth.py
@@ -6,7 +6,7 @@ import json
 import time
 from urllib.parse import unquote
 
-from duty_teller.i18n.lang import normalize_lang
+import duty_teller.config as config
 
 # Telegram algorithm: https://core.telegram.org/bots/webapps#validating-data-received-via-the-mini-app
 # Data-check string: sorted key=value with URL-decoded values, then HMAC-SHA256(WebAppData, token) as secret.
@@ -48,12 +48,12 @@ def validate_init_data_with_reason(
     Returns:
         Tuple (telegram_user_id, username, reason, lang). reason is one of: "ok",
         "empty", "no_hash", "hash_mismatch", "auth_date_expired", "no_user",
-        "user_invalid", "no_user_id". lang is from user.language_code normalized
-        to 'ru' or 'en'; 'en' when no user. On success: (user.id, username or None,
-        "ok", lang).
+        "user_invalid", "no_user_id". lang is always config.DEFAULT_LANGUAGE.
+        On success: (user.id, username or None, "ok", lang).
     """
+    lang = config.DEFAULT_LANGUAGE
     if not init_data or not bot_token:
-        return (None, None, "empty", "en")
+        return (None, None, "empty", lang)
     init_data = init_data.strip()
     params = {}
     for part in init_data.split("&"):
@@ -65,7 +65,7 @@ def validate_init_data_with_reason(
         params[key] = value
     hash_val = params.pop("hash", None)
     if not hash_val:
-        return (None, None, "no_hash", "en")
+        return (None, None, "no_hash", lang)
     data_pairs = sorted(params.items())
     # Data-check string: key=value with URL-decoded values (per Telegram example)
     data_string = "\n".join(f"{k}={unquote(v)}" for k, v in data_pairs)
@@ -81,27 +81,26 @@ def validate_init_data_with_reason(
         digestmod=hashlib.sha256,
     ).hexdigest()
     if not hmac.compare_digest(computed.lower(), hash_val.lower()):
-        return (None, None, "hash_mismatch", "en")
+        return (None, None, "hash_mismatch", lang)
     if max_age_seconds is not None and max_age_seconds > 0:
         auth_date_raw = params.get("auth_date")
         if not auth_date_raw:
-            return (None, None, "auth_date_expired", "en")
+            return (None, None, "auth_date_expired", lang)
         try:
             auth_date = int(float(auth_date_raw))
         except (ValueError, TypeError):
-            return (None, None, "auth_date_expired", "en")
+            return (None, None, "auth_date_expired", lang)
         if time.time() - auth_date > max_age_seconds:
-            return (None, None, "auth_date_expired", "en")
+            return (None, None, "auth_date_expired", lang)
     user_raw = params.get("user")
     if not user_raw:
-        return (None, None, "no_user", "en")
+        return (None, None, "no_user", lang)
     try:
         user = json.loads(unquote(user_raw))
     except (json.JSONDecodeError, TypeError):
-        return (None, None, "user_invalid", "en")
+        return (None, None, "user_invalid", lang)
     if not isinstance(user, dict):
-        return (None, None, "user_invalid", "en")
-    lang = normalize_lang(user.get("language_code"))
+        return (None, None, "user_invalid", lang)
     raw_id = user.get("id")
     if raw_id is None:
         return (None, None, "no_user_id", lang)
diff --git a/duty_teller/i18n/core.py b/duty_teller/i18n/core.py
index ddb332e..83b0fa4 100644
--- a/duty_teller/i18n/core.py
+++ b/duty_teller/i18n/core.py
@@ -1,9 +1,8 @@
-"""get_lang and t(): language from Telegram user, translate by key with fallback to en."""
+"""get_lang and t(): language from config (DEFAULT_LANGUAGE), translate by key with fallback to en."""
 
 from typing import TYPE_CHECKING
 
 import duty_teller.config as config
-from duty_teller.i18n.lang import normalize_lang
 from duty_teller.i18n.messages import MESSAGES
 
 if TYPE_CHECKING:
@@ -12,13 +11,12 @@ if TYPE_CHECKING:
 
 def get_lang(user: "User | None") -> str:
     """
-    Normalize Telegram user language to 'ru' or 'en'.
-    Uses normalize_lang for user.language_code; when user is None or has no
-    language_code, returns config.DEFAULT_LANGUAGE.
+    Return the application language: always config.DEFAULT_LANGUAGE.
+
+    The user argument is kept for backward compatibility but is ignored.
+    The whole deployment uses a single language from DEFAULT_LANGUAGE.
     """
-    if user is None or not getattr(user, "language_code", None):
-        return config.DEFAULT_LANGUAGE
-    return normalize_lang(user.language_code)
+    return config.DEFAULT_LANGUAGE
 
 
 def t(lang: str, key: str, **kwargs: str) -> str:
diff --git a/tests/test_api_dependencies.py b/tests/test_api_dependencies.py
index e68a2fd..599b10b 100644
--- a/tests/test_api_dependencies.py
+++ b/tests/test_api_dependencies.py
@@ -10,24 +10,27 @@ import duty_teller.config as config
 
 
 class TestLangFromAcceptLanguage:
-    """Tests for _lang_from_accept_language."""
+    """Tests for _lang_from_accept_language: always returns config.DEFAULT_LANGUAGE."""
 
-    def test_none_returns_default(self):
+    def test_always_returns_default_language(self):
+        """Header is ignored; result is always config.DEFAULT_LANGUAGE."""
         assert deps._lang_from_accept_language(None) == config.DEFAULT_LANGUAGE
-
-    def test_empty_string_returns_default(self):
         assert deps._lang_from_accept_language("") == config.DEFAULT_LANGUAGE
         assert deps._lang_from_accept_language("   ") == config.DEFAULT_LANGUAGE
+        assert deps._lang_from_accept_language("ru-RU,ru;q=0.9") == config.DEFAULT_LANGUAGE
+        assert deps._lang_from_accept_language("en-US") == config.DEFAULT_LANGUAGE
+        assert deps._lang_from_accept_language("zz") == config.DEFAULT_LANGUAGE
+        assert deps._lang_from_accept_language("x") == config.DEFAULT_LANGUAGE
 
-    def test_ru_ru_returns_ru(self):
-        assert deps._lang_from_accept_language("ru-RU,ru;q=0.9") == "ru"
+    def test_returns_ru_when_default_language_is_ru(self):
+        with patch.object(config, "DEFAULT_LANGUAGE", "ru"):
+            assert deps._lang_from_accept_language("en-US") == "ru"
+            assert deps._lang_from_accept_language(None) == "ru"
 
-    def test_en_us_returns_en(self):
-        assert deps._lang_from_accept_language("en-US") == "en"
-
-    def test_invalid_fallback_to_en(self):
-        assert deps._lang_from_accept_language("zz") == "en"
-        assert deps._lang_from_accept_language("x") == "en"
+    def test_returns_en_when_default_language_is_en(self):
+        with patch.object(config, "DEFAULT_LANGUAGE", "en"):
+            assert deps._lang_from_accept_language("ru-RU") == "en"
+            assert deps._lang_from_accept_language(None) == "en"
 
 
 class TestAuthErrorDetail:
diff --git a/tests/test_app.py b/tests/test_app.py
index 1281f15..c78d202 100644
--- a/tests/test_app.py
+++ b/tests/test_app.py
@@ -23,6 +23,41 @@ def test_health(client):
     assert r.json() == {"status": "ok"}
 
 
+def test_health_has_vary_accept_language(client):
+    """NoCacheStaticMiddleware adds Vary: Accept-Language to all responses."""
+    r = client.get("/health")
+    assert r.status_code == 200
+    assert "accept-language" in r.headers.get("vary", "").lower()
+
+
+def test_app_static_has_no_store_and_vary(client):
+    """Static files under /app get Cache-Control: no-store and Vary: Accept-Language."""
+    r = client.get("/app/")
+    if r.status_code != 200:
+        r = client.get("/app")
+    assert r.status_code == 200, "webapp static mount should serve index at /app or /app/"
+    assert r.headers.get("cache-control") == "no-store"
+    assert "accept-language" in r.headers.get("vary", "").lower()
+
+
+def test_app_js_has_no_store(client):
+    """JS and all static under /app get Cache-Control: no-store."""
+    r = client.get("/app/js/main.js")
+    assert r.status_code == 200
+    assert r.headers.get("cache-control") == "no-store"
+
+
+def test_app_config_js_returns_lang_from_default_language(client):
+    """GET /app/config.js returns JS setting window.__DT_LANG from config.DEFAULT_LANGUAGE."""
+    r = client.get("/app/config.js")
+    assert r.status_code == 200
+    assert r.headers.get("content-type", "").startswith("application/javascript")
+    assert r.headers.get("cache-control") == "no-store"
+    body = r.text
+    assert "window.__DT_LANG" in body
+    assert config.DEFAULT_LANGUAGE in body
+
+
 def test_duties_invalid_date_format(client):
     r = client.get("/api/duties", params={"from": "2025-01-01", "to": "invalid"})
     assert r.status_code == 400
diff --git a/tests/test_i18n.py b/tests/test_i18n.py
index 8a6db5d..6e8df5a 100644
--- a/tests/test_i18n.py
+++ b/tests/test_i18n.py
@@ -1,48 +1,46 @@
 """Unit tests for duty_teller.i18n: get_lang, t, fallback to en."""
 
-from unittest.mock import MagicMock
-
+from unittest.mock import MagicMock, patch
 
+import duty_teller.config as config
 from duty_teller.i18n import get_lang, t
 
 
-def test_get_lang_none_returns_en():
-    assert get_lang(None) == "en"
+def test_get_lang_always_returns_default_language():
+    """get_lang ignores user and always returns config.DEFAULT_LANGUAGE."""
+    assert get_lang(None) == config.DEFAULT_LANGUAGE
+    user_ru = MagicMock()
+    user_ru.language_code = "ru"
+    assert get_lang(user_ru) == config.DEFAULT_LANGUAGE
+    user_en = MagicMock()
+    user_en.language_code = "en"
+    assert get_lang(user_en) == config.DEFAULT_LANGUAGE
+    user_any = MagicMock(spec=[])
+    assert get_lang(user_any) == config.DEFAULT_LANGUAGE
 
 
-def test_get_lang_ru_returns_ru():
-    user = MagicMock()
-    user.language_code = "ru"
-    assert get_lang(user) == "ru"
+def test_get_lang_returns_ru_when_default_language_is_ru():
+    """When DEFAULT_LANGUAGE is ru, get_lang returns 'ru' regardless of user."""
+    with patch("duty_teller.i18n.core.config") as mock_cfg:
+        mock_cfg.DEFAULT_LANGUAGE = "ru"
+        from duty_teller.i18n.core import get_lang as core_get_lang
+
+        assert core_get_lang(None) == "ru"
+        user = MagicMock()
+        user.language_code = "en"
+        assert core_get_lang(user) == "ru"
 
 
-def test_get_lang_ru_ru_returns_ru():
-    user = MagicMock()
-    user.language_code = "ru-RU"
-    assert get_lang(user) == "ru"
+def test_get_lang_returns_en_when_default_language_is_en():
+    """When DEFAULT_LANGUAGE is en, get_lang returns 'en' regardless of user."""
+    with patch("duty_teller.i18n.core.config") as mock_cfg:
+        mock_cfg.DEFAULT_LANGUAGE = "en"
+        from duty_teller.i18n.core import get_lang as core_get_lang
 
-
-def test_get_lang_en_returns_en():
-    user = MagicMock()
-    user.language_code = "en"
-    assert get_lang(user) == "en"
-
-
-def test_get_lang_uk_returns_en():
-    user = MagicMock()
-    user.language_code = "uk"
-    assert get_lang(user) == "en"
-
-
-def test_get_lang_empty_returns_en():
-    user = MagicMock()
-    user.language_code = ""
-    assert get_lang(user) == "en"
-
-
-def test_get_lang_missing_attr_returns_en():
-    user = MagicMock(spec=[])  # no language_code
-    assert get_lang(user) == "en"
+        assert core_get_lang(None) == "en"
+        user = MagicMock()
+        user.language_code = "ru"
+        assert core_get_lang(user) == "en"
 
 
 def test_t_en_start_greeting():
diff --git a/tests/test_telegram_auth.py b/tests/test_telegram_auth.py
index 0820b3d..663b81a 100644
--- a/tests/test_telegram_auth.py
+++ b/tests/test_telegram_auth.py
@@ -1,5 +1,8 @@
 """Tests for duty_teller.api.telegram_auth.validate_init_data and validate_init_data_with_reason."""
 
+from unittest.mock import patch
+
+import duty_teller.config as config
 from duty_teller.api.telegram_auth import (
     validate_init_data,
     validate_init_data_with_reason,
@@ -52,7 +55,7 @@ def test_user_without_username_returns_none_from_validate_init_data():
 
 
 def test_user_without_username_but_with_id_succeeds_with_reason():
-    """With validate_init_data_with_reason, valid user.id is enough; username may be None."""
+    """With validate_init_data_with_reason, valid user.id is enough; lang is DEFAULT_LANGUAGE."""
     bot_token = "123:ABC"
     user = {"id": 456, "first_name": "Test", "language_code": "ru"}
     init_data = make_init_data(user, bot_token)
@@ -62,11 +65,11 @@ def test_user_without_username_but_with_id_succeeds_with_reason():
     assert telegram_user_id == 456
     assert username is None
     assert reason == "ok"
-    assert lang == "ru"
+    assert lang == config.DEFAULT_LANGUAGE
 
 
 def test_user_without_id_returns_no_user_id():
-    """When user object exists but has no 'id', return no_user_id."""
+    """When user object exists but has no 'id', return no_user_id; lang is DEFAULT_LANGUAGE."""
     bot_token = "123:ABC"
     user = {"first_name": "Test"}  # no id
     init_data = make_init_data(user, bot_token)
@@ -76,7 +79,17 @@ def test_user_without_id_returns_no_user_id():
     assert telegram_user_id is None
     assert username is None
     assert reason == "no_user_id"
-    assert lang == "en"
+    assert lang == config.DEFAULT_LANGUAGE
+
+
+def test_validate_init_data_with_reason_returns_default_language_ignoring_user_lang():
+    """Returned lang is always config.DEFAULT_LANGUAGE, not user.language_code."""
+    with patch("duty_teller.api.telegram_auth.config.DEFAULT_LANGUAGE", "ru"):
+        user = {"id": 1, "first_name": "U", "language_code": "en"}
+        init_data = make_init_data(user, "123:ABC")
+        _, _, reason, lang = validate_init_data_with_reason(init_data, "123:ABC")
+        assert reason == "ok"
+        assert lang == "ru"
 
 
 def test_empty_init_data_returns_none():
diff --git a/webapp/css/calendar.css b/webapp/css/calendar.css
index f6036d0..6551267 100644
--- a/webapp/css/calendar.css
+++ b/webapp/css/calendar.css
@@ -152,6 +152,19 @@
   border: 1px solid color-mix(in srgb, var(--bg) 50%, transparent);
 }
 
+@media (hover: hover) {
+  .day.holiday:hover {
+    background: linear-gradient(
+      135deg,
+      color-mix(in srgb, var(--accent) 12%, var(--surface)) 0%,
+      color-mix(in srgb, var(--today) 22%, transparent) 100%
+    );
+  }
+  .day.today.holiday:hover {
+    background: color-mix(in srgb, var(--bg) 15%, var(--today));
+  }
+}
+
 .day {
   cursor: pointer;
 }
diff --git a/webapp/index.html b/webapp/index.html
index addfab1..f2b11a8 100644
--- a/webapp/index.html
+++ b/webapp/index.html
@@ -36,15 +36,66 @@
     <div id="currentDutyView" class="current-duty-view hidden"></div>
   </div>
   <script src="https://telegram.org/js/telegram-web-app.js"></script>
+  <script>
+    if (window.Telegram && window.Telegram.WebApp && window.Telegram.WebApp.ready) {
+      window.Telegram.WebApp.ready();
+    }
+  </script>
+  <script src="/app/config.js"></script>
   <script type="importmap">
   {
     "scopes": {
       "./js/": {
-        "./js/i18n.js": "./js/i18n.js?v=1"
+        "./js/api.js": "./js/api.js?v=3",
+        "./js/auth.js": "./js/auth.js?v=3",
+        "./js/calendar.js": "./js/calendar.js?v=2",
+        "./js/constants.js": "./js/constants.js?v=2",
+        "./js/contactHtml.js": "./js/contactHtml.js?v=2",
+        "./js/currentDuty.js": "./js/currentDuty.js?v=2",
+        "./js/dateUtils.js": "./js/dateUtils.js?v=2",
+        "./js/dayDetail.js": "./js/dayDetail.js?v=2",
+        "./js/dom.js": "./js/dom.js?v=3",
+        "./js/dutyList.js": "./js/dutyList.js?v=2",
+        "./js/hints.js": "./js/hints.js?v=2",
+        "./js/i18n.js": "./js/i18n.js?v=3",
+        "./js/theme.js": "./js/theme.js?v=2",
+        "./js/ui.js": "./js/ui.js?v=2",
+        "./js/utils.js": "./js/utils.js?v=2"
       }
     }
   }
   </script>
-  <script type="module" src="js/main.js?v=4"></script>
+  <script type="module" src="js/main.js?v=5" id="main-module"></script>
+  <script>
+    (function() {
+      var loadTimeout = 10000;
+      var mainScript = document.getElementById("main-module");
+      if (mainScript) {
+        mainScript.addEventListener("error", function() {
+          var loading = document.getElementById("loading");
+          if (loading && !loading.classList.contains("hidden")) {
+            loading.classList.add("hidden");
+            var err = document.getElementById("error");
+            if (err) {
+              err.hidden = false;
+              err.textContent = "Failed to load app. Check connection and try again.";
+            }
+          }
+        });
+      }
+      setTimeout(function() {
+        if (window.__dtReady) return;
+        var loading = document.getElementById("loading");
+        if (loading && !loading.classList.contains("hidden")) {
+          loading.classList.add("hidden");
+          var err = document.getElementById("error");
+          if (err) {
+            err.hidden = false;
+            err.textContent = "App is taking too long to load. Check your connection and refresh.";
+          }
+        }
+      }, loadTimeout);
+    })();
+  </script>
 </body>
 </html>
diff --git a/webapp/js/api.js b/webapp/js/api.js
index 4204988..c7d2d1d 100644
--- a/webapp/js/api.js
+++ b/webapp/js/api.js
@@ -17,7 +17,7 @@ import { t } from "./i18n.js";
 export function buildFetchOptions(initData, externalSignal) {
   const headers = {};
   if (initData) headers["X-Telegram-Init-Data"] = initData;
-  headers["Accept-Language"] = state.lang || "ru";
+  headers["Accept-Language"] = state.lang || "en";
   const controller = new AbortController();
   const timeoutId = setTimeout(() => controller.abort(), FETCH_TIMEOUT_MS);
   const onAbort = () => controller.abort();
diff --git a/webapp/js/auth.js b/webapp/js/auth.js
index 9bed5fa..1f5dd10 100644
--- a/webapp/js/auth.js
+++ b/webapp/js/auth.js
@@ -34,23 +34,15 @@ export function getInitData() {
   if (hash) {
     const fromHash = getTgWebAppDataFromHash(hash);
     if (fromHash) return fromHash;
-    try {
-      const hashParams = new URLSearchParams(hash);
-      const tgFromHash = hashParams.get("tgWebAppData");
-      if (tgFromHash) return decodeURIComponent(tgFromHash);
-    } catch (e) {
-      /* ignore */
-    }
+    const hashParams = new URLSearchParams(hash);
+    const tgFromHash = hashParams.get("tgWebAppData");
+    if (tgFromHash) return tgFromHash;
   }
   const q = window.location.search
     ? new URLSearchParams(window.location.search).get("tgWebAppData")
     : null;
   if (q) {
-    try {
-      return decodeURIComponent(q);
-    } catch (e) {
-      return q;
-    }
+    return q;
   }
   return "";
 }
diff --git a/webapp/js/auth.test.js b/webapp/js/auth.test.js
index 8b1d1aa..364fd31 100644
--- a/webapp/js/auth.test.js
+++ b/webapp/js/auth.test.js
@@ -66,6 +66,28 @@ describe("getInitData", () => {
     window.location = { ...origLocation, hash: "", search: "" };
     expect(getInitData()).toBe("");
   });
+
+  it("returns data from hash query param without double-decoding", () => {
+    window.Telegram = { WebApp: { initData: "" } };
+    delete window.location;
+    window.location = {
+      ...origLocation,
+      hash: "#tgWebAppVersion=6&tgWebAppData=user%3Dname%26hash%3Dabc",
+      search: "",
+    };
+    expect(getInitData()).toBe("user=name&hash=abc");
+  });
+
+  it("returns data from search query param without double-decoding", () => {
+    window.Telegram = { WebApp: { initData: "" } };
+    delete window.location;
+    window.location = {
+      ...origLocation,
+      hash: "",
+      search: "?tgWebAppData=user%3Dname%26hash%3Dabc",
+    };
+    expect(getInitData()).toBe("user=name&hash=abc");
+  });
 });
 
 describe("isLocalhost", () => {
diff --git a/webapp/js/dom.js b/webapp/js/dom.js
index 6970843..fc2d857 100644
--- a/webapp/js/dom.js
+++ b/webapp/js/dom.js
@@ -67,7 +67,7 @@ export const state = {
   /** @type {ReturnType<typeof setInterval>|null} */
   todayRefreshInterval: null,
   /** @type {'ru'|'en'} */
-  lang: "ru",
+  lang: "en",
   /** One-time bind flag for sticky scroll shadow listener. */
   stickyScrollBound: false,
   /** One-time bind flag for calendar (info button) hint document listeners. */
diff --git a/webapp/js/i18n.js b/webapp/js/i18n.js
index 6ceb7e0..724cea5 100644
--- a/webapp/js/i18n.js
+++ b/webapp/js/i18n.js
@@ -1,9 +1,7 @@
 /**
- * Internationalization: language detection and translations for webapp.
+ * Internationalization: language from backend config (window.__DT_LANG) and translations.
  */
 
-import { getInitData } from "./auth.js";
-
 /** @type {Record<string, Record<string, string>>} */
 export const MESSAGES = {
   en: {
@@ -135,34 +133,25 @@ const WEEKDAY_KEYS = [
  * @param {string} code - e.g. 'ru', 'en', 'uk'
  * @returns {'ru'|'en'}
  */
-function normalizeLang(code) {
-  if (!code || typeof code !== "string") return "ru";
+export function normalizeLang(code) {
+  if (!code || typeof code !== "string") return "en";
   const lower = code.toLowerCase();
   if (lower.startsWith("ru")) return "ru";
   return "en";
 }
 
 /**
- * Detect language: Telegram initData user.language_code → navigator → fallback 'ru'.
+ * Get application language from backend config (window.__DT_LANG).
+ * Set by /app/config.js from DEFAULT_LANGUAGE. Fallback to 'en' if missing or invalid.
  * @returns {'ru'|'en'}
  */
 export function getLang() {
-  const initData = getInitData();
-  if (initData) {
-    try {
-      const params = new URLSearchParams(initData);
-      const userStr = params.get("user");
-      if (userStr) {
-        const user = JSON.parse(decodeURIComponent(userStr));
-        const code = user && user.language_code;
-        if (code) return normalizeLang(code);
-      }
-    } catch (e) {
-      /* ignore */
-    }
-  }
-  const nav = navigator.language || (navigator.languages && navigator.languages[0]) || "";
-  return normalizeLang(nav);
+  const raw =
+    typeof window !== "undefined" && window.__DT_LANG != null
+      ? String(window.__DT_LANG)
+      : "";
+  const lang = normalizeLang(raw);
+  return lang === "ru" ? "ru" : "en";
 }
 
 /**
diff --git a/webapp/js/i18n.test.js b/webapp/js/i18n.test.js
index 36380d6..8846f7c 100644
--- a/webapp/js/i18n.test.js
+++ b/webapp/js/i18n.test.js
@@ -1,50 +1,76 @@
 /**
- * Unit tests for i18n: getLang, t (fallback, params), monthName.
+ * Unit tests for i18n: getLang (window.__DT_LANG), normalizeLang, t (fallback, params), monthName.
  */
 
-import { describe, it, expect, vi, beforeEach } from "vitest";
+import { describe, it, expect, beforeEach, afterEach } from "vitest";
 
-const mockGetInitData = vi.fn();
-vi.mock("./auth.js", () => ({ getInitData: () => mockGetInitData() }));
-
-import { getLang, t, monthName, MESSAGES } from "./i18n.js";
+import { getLang, normalizeLang, t, monthName, MESSAGES } from "./i18n.js";
 
 describe("getLang", () => {
-  const origNavigator = globalThis.navigator;
+  const orig__DT_LANG = globalThis.window?.__DT_LANG;
 
-  beforeEach(() => {
-    mockGetInitData.mockReset();
+  afterEach(() => {
+    if (typeof globalThis.window !== "undefined") {
+      if (orig__DT_LANG !== undefined) {
+        globalThis.window.__DT_LANG = orig__DT_LANG;
+      } else {
+        delete globalThis.window.__DT_LANG;
+      }
+    }
   });
 
-  it("returns lang from initData user when present", () => {
-    mockGetInitData.mockReturnValue(
-      "user=" + encodeURIComponent(JSON.stringify({ language_code: "en" }))
-    );
-    expect(getLang()).toBe("en");
-  });
-
-  it("normalizes ru from initData", () => {
-    mockGetInitData.mockReturnValue(
-      "user=" + encodeURIComponent(JSON.stringify({ language_code: "ru" }))
-    );
+  it("returns ru when window.__DT_LANG is ru", () => {
+    globalThis.window.__DT_LANG = "ru";
     expect(getLang()).toBe("ru");
   });
 
-  it("falls back to navigator.language when initData empty", () => {
-    mockGetInitData.mockReturnValue("");
-    Object.defineProperty(globalThis, "navigator", {
-      value: { ...origNavigator, language: "en-US", languages: ["en-US", "en"] },
-      configurable: true,
-    });
+  it("returns en when window.__DT_LANG is en", () => {
+    globalThis.window.__DT_LANG = "en";
     expect(getLang()).toBe("en");
   });
 
-  it("normalizes to en for unknown language code", () => {
-    mockGetInitData.mockReturnValue(
-      "user=" + encodeURIComponent(JSON.stringify({ language_code: "uk" }))
-    );
+  it("returns en when window.__DT_LANG is missing", () => {
+    delete globalThis.window.__DT_LANG;
     expect(getLang()).toBe("en");
   });
+
+  it("returns en when window.__DT_LANG is invalid (unknown code)", () => {
+    globalThis.window.__DT_LANG = "uk";
+    expect(getLang()).toBe("en");
+  });
+
+  it("returns ru when window.__DT_LANG is ru-RU (normalized)", () => {
+    globalThis.window.__DT_LANG = "ru-RU";
+    expect(getLang()).toBe("ru");
+  });
+
+  it("returns en when window.__DT_LANG is empty string", () => {
+    globalThis.window.__DT_LANG = "";
+    expect(getLang()).toBe("en");
+  });
+
+  it("returns en when window.__DT_LANG is null", () => {
+    globalThis.window.__DT_LANG = null;
+    expect(getLang()).toBe("en");
+  });
+});
+
+describe("normalizeLang", () => {
+  it("returns ru for ru-like codes", () => {
+    expect(normalizeLang("ru")).toBe("ru");
+    expect(normalizeLang("ru-RU")).toBe("ru");
+  });
+
+  it("returns en for en and others", () => {
+    expect(normalizeLang("en")).toBe("en");
+    expect(normalizeLang("en-US")).toBe("en");
+    expect(normalizeLang("uk")).toBe("en");
+  });
+
+  it("returns en for empty or invalid", () => {
+    expect(normalizeLang("")).toBe("en");
+    expect(normalizeLang(null)).toBe("en");
+  });
 });
 
 describe("t", () => {
diff --git a/webapp/js/main.js b/webapp/js/main.js
index c90587d..6fafc7a 100644
--- a/webapp/js/main.js
+++ b/webapp/js/main.js
@@ -37,23 +37,35 @@ import {
 initTheme();
 
 state.lang = getLang();
-document.documentElement.lang = state.lang;
-document.title = t(state.lang, "app.title");
-const loadingEl = getLoadingEl();
-const loadingTextEl = loadingEl ? loadingEl.querySelector(".loading__text") : null;
-if (loadingTextEl) loadingTextEl.textContent = t(state.lang, "loading");
-const dayLabels = weekdayLabels(state.lang);
-const weekdaysEl = getWeekdaysEl();
-if (weekdaysEl) {
-  const spans = weekdaysEl.querySelectorAll("span");
-  spans.forEach((span, i) => {
-    if (dayLabels[i]) span.textContent = dayLabels[i];
-  });
+
+/**
+ * Apply current state.lang to document and locale-dependent UI elements (title, loading, weekdays, nav).
+ * Call at startup and after re-evaluating lang when initData becomes available.
+ * Exported for tests.
+ */
+export function applyLangToUi() {
+  document.documentElement.lang = state.lang;
+  document.title = t(state.lang, "app.title");
+  const loadingEl = getLoadingEl();
+  const loadingTextEl = loadingEl ? loadingEl.querySelector(".loading__text") : null;
+  if (loadingTextEl) loadingTextEl.textContent = t(state.lang, "loading");
+  const dayLabels = weekdayLabels(state.lang);
+  const weekdaysEl = getWeekdaysEl();
+  if (weekdaysEl) {
+    const spans = weekdaysEl.querySelectorAll("span");
+    spans.forEach((span, i) => {
+      if (dayLabels[i]) span.textContent = dayLabels[i];
+    });
+  }
+  const prevBtn = getPrevBtn();
+  const nextBtn = getNextBtn();
+  if (prevBtn) prevBtn.setAttribute("aria-label", t(state.lang, "nav.prev_month"));
+  if (nextBtn) nextBtn.setAttribute("aria-label", t(state.lang, "nav.next_month"));
 }
-const prevBtn = getPrevBtn();
-const nextBtn = getNextBtn();
-if (prevBtn) prevBtn.setAttribute("aria-label", t(state.lang, "nav.prev_month"));
-if (nextBtn) nextBtn.setAttribute("aria-label", t(state.lang, "nav.next_month"));
+
+applyLangToUi();
+
+window.__dtReady = true;
 
 /**
  * Run callback when Telegram WebApp is ready (or immediately outside Telegram).
@@ -62,9 +74,6 @@ if (nextBtn) nextBtn.setAttribute("aria-label", t(state.lang, "nav.next_month"))
  */
 function runWhenReady(cb) {
   if (window.Telegram && window.Telegram.WebApp) {
-    if (window.Telegram.WebApp.ready) {
-      window.Telegram.WebApp.ready();
-    }
     if (window.Telegram.WebApp.expand) {
       window.Telegram.WebApp.expand();
     }
@@ -193,6 +202,8 @@ async function loadMonth() {
     setNavEnabled(true);
     return;
   }
+  const errorEl2 = getErrorEl();
+  if (errorEl2) errorEl2.hidden = true;
   const loading = getLoadingEl();
   if (loading) loading.classList.add("hidden");
   setNavEnabled(true);
@@ -276,6 +287,11 @@ function bindStickyScrollShadow() {
 
 runWhenReady(() => {
   requireTelegramOrLocalhost(() => {
+    const newLang = getLang();
+    if (newLang !== state.lang) {
+      state.lang = newLang;
+      applyLangToUi();
+    }
     bindStickyScrollShadow();
     initDayDetail();
     initHints();
@@ -284,7 +300,6 @@ runWhenReady(() => {
         window.Telegram.WebApp.initDataUnsafe.start_param) ||
       "";
     if (startParam === "duty") {
-      state.lang = getLang();
       showCurrentDutyView(() => {
         hideCurrentDutyView();
         loadMonth();
diff --git a/webapp/js/main.test.js b/webapp/js/main.test.js
new file mode 100644
index 0000000..52cec7c
--- /dev/null
+++ b/webapp/js/main.test.js
@@ -0,0 +1,69 @@
+/**
+ * Unit tests for main.js: applyLangToUi (locale-to-DOM) and lang re-evaluation behaviour.
+ */
+
+import { describe, it, expect, beforeAll } from "vitest";
+
+beforeAll(() => {
+  document.body.innerHTML =
+    '<div id="calendarSticky">' +
+    '<button id="prevMonth"></button><h1 id="monthTitle"></h1><button id="nextMonth"></button>' +
+    '<div class="weekdays">' +
+    '<span></span><span></span><span></span><span></span><span></span><span></span><span></span>' +
+    "</div></div>" +
+    '<div id="dutyList"></div>' +
+    '<div id="loading"><span class="loading__text"></span></div>' +
+    '<div id="error"></div><div id="accessDenied"></div>';
+});
+
+import { applyLangToUi } from "./main.js";
+import { state } from "./dom.js";
+
+describe("applyLangToUi", () => {
+  it("state.lang is set from getLang() at startup (getLang reads window.__DT_LANG; default en)", () => {
+    expect(state.lang).toBe("en");
+  });
+
+  it("sets document title and loading text for en", () => {
+    state.lang = "en";
+    applyLangToUi();
+    expect(document.title).toBe("Duty Calendar");
+    const loadingText = document.querySelector(".loading__text");
+    expect(loadingText && loadingText.textContent).toBe("Loading…");
+  });
+
+  it("sets document title and loading text for ru", () => {
+    state.lang = "ru";
+    applyLangToUi();
+    expect(document.title).toBe("Календарь дежурств");
+    const loadingText = document.querySelector(".loading__text");
+    expect(loadingText && loadingText.textContent).toBe("Загрузка…");
+  });
+
+  it("sets documentElement.lang to state.lang", () => {
+    state.lang = "en";
+    applyLangToUi();
+    expect(document.documentElement.lang).toBe("en");
+    state.lang = "ru";
+    applyLangToUi();
+    expect(document.documentElement.lang).toBe("ru");
+  });
+
+  it("sets weekday labels and nav aria-labels for current lang", () => {
+    state.lang = "en";
+    applyLangToUi();
+    const weekdays = document.querySelectorAll(".weekdays span");
+    expect(weekdays.length).toBe(7);
+    expect(weekdays[0].textContent).toBe("Mon");
+    const prevBtn = document.getElementById("prevMonth");
+    const nextBtn = document.getElementById("nextMonth");
+    expect(prevBtn && prevBtn.getAttribute("aria-label")).toBe("Previous month");
+    expect(nextBtn && nextBtn.getAttribute("aria-label")).toBe("Next month");
+
+    state.lang = "ru";
+    applyLangToUi();
+    expect(weekdays[0].textContent).toBe("Пн");
+    expect(prevBtn && prevBtn.getAttribute("aria-label")).toContain("Пред");
+    expect(nextBtn && nextBtn.getAttribute("aria-label")).toContain("След");
+  });
+});