Test: i18n routing (lang param / fallback / redirects)

[wab]

Parent

#121

What to build

Tests for i18n routing behavior — ($lang) URL param handling, default-language fallback, and lang-switch redirects.

Acceptance criteria

• Default route (no lang prefix) resolves to configured default lang
• /en/... routes load English translations
• Unknown lang prefix falls back to default
• Lang switch preserves the current path
• ≥ 8 test cases

Blocked by

None - can start immediately