How to Test Multi-Language Support in Vibecoded Apps

Switch your app to each supported language and navigate every page. Look for untranslated strings — text that remains in the default language when it should be translated. AI-generated i18n implementations frequently miss strings in error messages, validation text, confirmation dialogs, date relative labels ('2 days ago'), and dynamically generated content.

Check for hardcoded strings in components that bypass the translation system entirely. Search your codebase for string literals in JSX or template files that should use translation keys. Also test that pluralization works correctly: '1 item' vs '2 items' requires special handling in many languages, and AI-generated code often gets this wrong.

German text is typically 30% longer than English, while Chinese and Japanese text is often more compact. Switch to your longest-text language and check that no text overflows containers, gets cut off, or breaks layouts. Buttons should expand to fit their labels or use ellipsis gracefully. AI-generated fixed-width layouts are especially prone to this issue.

Test right-to-left (RTL) languages like Arabic and Hebrew if you support them. The entire layout should mirror: navigation on the right, text aligned right, progress bars filling from right to left. This is one of the most commonly broken features in vibecoded apps because it requires CSS logical properties (margin-inline-start instead of margin-left) that AI tools rarely generate.

Verify that dates, numbers, and currencies are formatted according to the user's locale. The date 3/14/2026 is March 14 in US format but invalid or 3 February in other formats. AI-generated code almost always hardcodes US date and number formats. Use the browser's Intl API or a library like date-fns with locale support.

Test currency formatting across locales: USD $1,234.56 in the US becomes 1.234,56 $ in Germany (with comma as decimal separator and period as thousands separator). Also test time zones if your app displays times — a scheduled event should show the correct local time for each user.