test(i18n): add comprehensive tests and documentation for i18n system

- Add 45 tests covering translation files, translate(), Accept-Language
  parsing, language detection, l10n formatters, and API endpoints
- Create InternationalizationGuide.md documentation
- Fix linting issues (E741, PLW2901)

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

app/api/i18n.py

@@ -24,7 +24,7 @@ from app.utils.i18n import (
 
 logger = logging.getLogger(__name__)
 
-router = APIRouter(prefix="/api/i18n", tags=["i18n"])
+router = APIRouter(prefix="/i18n", tags=["i18n"])
 
 
 class LanguageInfo(BaseModel):
@@ -103,7 +103,7 @@ async def set_language(
     _persist_language_to_profile(request, db, lang)
 
     language_name = next(
-        (l["native"] for l in SUPPORTED_LANGUAGES if l["code"] == lang),
+        (entry["native"] for entry in SUPPORTED_LANGUAGES if entry["code"] == lang),
         lang,
     )
     logger.info("Language preference set to '%s'", lang)

app/utils/i18n.py

@@ -225,8 +225,8 @@ def _parse_accept_language(header: str) -> str | None:
         return None
 
     entries: list[tuple[float, str]] = []
-    for part in header.split(","):
-        part = part.strip()
+    for raw_part in header.split(","):
+        part = raw_part.strip()
         if not part:
             continue
         if ";q=" in part: