feat: unify language handling across the application

- Updated the language configuration to use a single source of truth from `DEFAULT_LANGUAGE` for the bot, API, and Mini App, eliminating auto-detection from user settings.
- Refactored the `get_lang` function to always return `DEFAULT_LANGUAGE`, ensuring consistent language usage throughout the application.
- Modified the handling of language in various components, including API responses and UI elements, to reflect the new language management approach.
- Enhanced documentation and comments to clarify the changes in language handling.
- Added unit tests to verify the new language handling behavior and ensure coverage for the updated functionality.

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: