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/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: