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/i18n/core.py

@@ -0,0 +1,37 @@
+"""get_lang and t(): language from Telegram user, translate by key with fallback to en."""
+
+from typing import TYPE_CHECKING
+
+import duty_teller.config as config
+from duty_teller.i18n.messages import MESSAGES
+
+if TYPE_CHECKING:
+    from telegram import User
+
+
+def get_lang(user: "User | None") -> str:
+    """
+    Normalize Telegram user language to 'ru' or 'en'.
+    If user has language_code starting with 'ru' (e.g. ru, ru-RU) return 'ru', else 'en'.
+    When user is None or has no language_code, return config.DEFAULT_LANGUAGE.
+    """
+    if user is None or not getattr(user, "language_code", None):
+        return config.DEFAULT_LANGUAGE
+    code = (user.language_code or "").strip().lower()
+    return "ru" if code.startswith("ru") else "en"
+
+
+def t(lang: str, key: str, **kwargs: str) -> str:
+    """
+    Return translated string for lang and key; substitute kwargs into placeholders like {phone}.
+    Fallback to 'en' if key missing for lang.
+    """
+    lang = "ru" if lang == "ru" else "en"
+    messages = MESSAGES.get(lang) or MESSAGES["en"]
+    template = messages.get(key)
+    if template is None:
+        template = MESSAGES["en"].get(key, key)
+    try:
+        return template.format(**kwargs)
+    except KeyError:
+        return template