Internationalization (i18n) i18next (React — recommended) Translation Files next-intl (Next.js) Date & Number Formatting ICU Message Format Translation File Structure Anti-Patterns | Anti-Pattern | Fix | |--------------|-----| | Hardcoded strings in UI | Extract all user-facing text to translation files | | String concatenation for sentences | Use interpolation ( ) or ICU format | | Manual pluralization logic | Use built-in plural rules ( , ) | | Date formatting with string manipulation | Use | | No fallback language | Always set | | Giant single translation file | Split by namespace (common,…