Add Multi-language Support (i18n) for Backend and Frontend

back/app/language_service.py

@@ -2,6 +2,8 @@
 Language service for normalizing language codes and managing translations.
 """
 
+from fastapi import Query, Header
+
 SUPPORTED_LANGUAGES = [
     "en",  # English
     "es",  # Spanish
@@ -50,3 +52,38 @@ def normalize_language_code(lang: str) -> str | None:
 def get_supported_languages() -> list[str]:
     """Return list of supported language codes."""
     return SUPPORTED_LANGUAGES.copy()
+
+
+def get_requested_language(
+    lang: str | None = Query(None, description="Language code (e.g. en, es, zh-CN)"),
+    accept_language: str | None = Header(
+        None, alias="Accept-Language", description="Accept-Language header"
+    ),
+) -> str:
+    """
+    Determine the requested language based on query param or Accept-Language header.
+    Returns normalized language code or 'en' as fallback.
+    """
+    # 1. Check explicit lang query parameter
+    if lang:
+        normalized = normalize_language_code(lang)
+        if normalized:
+            return normalized
+
+    # 2. Parse Accept-Language header
+    if accept_language:
+        # Simple parsing - take the first language with highest quality
+        # Format: "en-US,en;q=0.9,es;q=0.8"
+        languages = []
+        for part in accept_language.split(","):
+            lang_part = part.strip().split(";")[0].strip()
+            if lang_part:
+                normalized = normalize_language_code(lang_part)
+                if normalized:
+                    languages.append(normalized)
+
+        if languages:
+            return languages[0]  # Return highest priority
+
+    # 3. Fallback to English
+    return "en"