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/services/group_duty_pin_service.py

@@ -13,12 +13,13 @@ from duty_teller.db.repository import (
     delete_group_duty_pin,
     get_all_group_duty_pin_chat_ids,
 )
+from duty_teller.i18n import t
 
 
-def format_duty_message(duty, user, tz_name: str) -> str:
+def format_duty_message(duty, user, tz_name: str, lang: str = "en") -> str:
     """Build the text for the pinned message. duty, user may be None."""
     if duty is None or user is None:
-        return "Сейчас дежурства нет."
+        return t(lang, "duty.no_duty")
     try:
         tz = ZoneInfo(tz_name)
     except Exception:
@@ -39,8 +40,9 @@ def format_duty_message(duty, user, tz_name: str) -> str:
         f"{start_local.strftime('%d.%m.%Y %H:%M')} — "
         f"{end_local.strftime('%d.%m.%Y %H:%M')} ({tz_hint})"
     )
+    label = t(lang, "duty.label")
     lines = [
-        f"🕐 Дежурство: {time_range}",
+        f"🕐 {label} {time_range}",
         f"👤 {user.full_name}",
     ]
     if user.phone:
@@ -50,14 +52,14 @@ def format_duty_message(duty, user, tz_name: str) -> str:
     return "\n".join(lines)
 
 
-def get_duty_message_text(session: Session, tz_name: str) -> str:
+def get_duty_message_text(session: Session, tz_name: str, lang: str = "en") -> str:
     """Get current duty from DB and return formatted message."""
     now = datetime.now(timezone.utc)
     result = get_current_duty(session, now)
     if result is None:
-        return "Сейчас дежурства нет."
+        return t(lang, "duty.no_duty")
     duty, user = result
-    return format_duty_message(duty, user, tz_name)
+    return format_duty_message(duty, user, tz_name, lang)
 
 
 def get_next_shift_end_utc(session: Session) -> datetime | None: