Add internationalization support and enhance language handling

- Introduced a new i18n module for managing translations and language normalization, supporting both Russian and English.
- Updated various handlers and services to utilize the new translation functions for user-facing messages, improving user experience based on language preferences.
- Enhanced error handling and response messages to be language-aware, ensuring appropriate feedback is provided to users in their preferred language.
- Added tests for the i18n module to validate language detection and translation functionality.
- Updated the example environment file to include a default language configuration.

duty_teller/config.py

@@ -12,6 +12,14 @@ load_dotenv()
 PROJECT_ROOT = Path(__file__).resolve().parent.parent
 
 
+def _normalize_default_language(value: str) -> str:
+    """Normalize DEFAULT_LANGUAGE from env to 'ru' or 'en'."""
+    if not value:
+        return "en"
+    v = value.strip().lower()
+    return "ru" if v.startswith("ru") else "en"
+
+
 @dataclass(frozen=True)
 class Settings:
     """Optional injectable settings built from env. Tests can override or build from env."""
@@ -27,6 +35,7 @@ class Settings:
     cors_origins: list[str]
     external_calendar_ics_url: str
     duty_display_tz: str
+    default_language: str
 
     @classmethod
     def from_env(cls) -> "Settings":
@@ -62,6 +71,9 @@ class Settings:
             ).strip(),
             duty_display_tz=os.getenv("DUTY_DISPLAY_TZ", "Europe/Moscow").strip()
             or "Europe/Moscow",
+            default_language=_normalize_default_language(
+                os.getenv("DEFAULT_LANGUAGE", "en").strip()
+            ),
         )
 
 
@@ -95,6 +107,9 @@ EXTERNAL_CALENDAR_ICS_URL = os.getenv("EXTERNAL_CALENDAR_ICS_URL", "").strip()
 DUTY_DISPLAY_TZ = (
     os.getenv("DUTY_DISPLAY_TZ", "Europe/Moscow").strip() or "Europe/Moscow"
 )
+DEFAULT_LANGUAGE = _normalize_default_language(
+    os.getenv("DEFAULT_LANGUAGE", "en").strip()
+)
 
 
 def is_admin(username: str) -> bool: