i18n .use(Backend) .use(LanguageDetector) .use(initReactI18next) .init({ fallbackLng: 'en', ns: ['common', 'auth'], defaultNS: 'common', interpolation: { escapeValue: false }, });

export default i18n;


```tsx
// Usage in components
import { useTranslation } from 'react-i18next';

function Welcome() {
  const { t } = useTranslation();

  return (
    <>
      <h1>{t('welcome', { name: 'John' })}</h1>
      <p>{t('items_count', { count: 5 })}</p>
    </>
  );
}

Translation Files

// locales/en/common.json
{
  "welcome": "Welcome, {{name}}!",
  "items_count_one": "{{count}} item",
  "items_count_other": "{{count}} items"
}

// locales/it/common.json
{
  "welcome": "Benvenuto, {{name}}!",
  "items_count_one": "{{count}} elemento",
  "items_count_other": "{{count}} elementi"
}

next-intl (Next.js)

// i18n/request.ts
import { getRequestConfig } from 'next-intl/server';

export default getRequestConfig(async ({ locale }) => ({
  messages: (await import(`../messages/${locale}.json`)).default,
}));

import { useTranslations } from 'next-intl';

function Page() {
  const t = useTranslations('HomePage');
  return <h1>{t('title')}</h1>;
}

Date & Number Formatting

// Use Intl API (built-in, no library needed)
const dateFormatter = new Intl.DateTimeFormat('de-DE', {
  year: 'numeric', month: 'long', day: 'numeric',
});
dateFormatter.format(new Date()); // "5. Marz 2026"

const currencyFormatter = new Intl.NumberFormat('en-US', {
  style: 'currency', currency: 'USD',
});
currencyFormatter.format(42.5); // "$42.50"

// Relative time
const rtf = new Intl.RelativeTimeFormat('en', { numeric: 'auto' });
rtf.format(-1, 'day'); // "yesterday"

ICU Message Format

{
  "greeting": "{gender, select, male {Mr.} female {Ms.} other {Mx.}} {name}",
  "notifications": "{count, plural, =0 {No notifications} one {# notification} other {# notifications}}"
}

Translation File Structure

locales/
├── en/
│   ├── common.json      # Shared strings
│   ├── auth.json         # Auth-specific
│   └── errors.json       # Error messages
├── it/
│   ├── common.json
│   ├── auth.json
│   └── errors.json

Anti-Patterns

Anti-Pattern	Fix
Hardcoded strings in UI	Extract all user-facing text to translation files
String concatenation for sentences	Use interpolation (`{{name}}`) or ICU format
Manual pluralization logic	Use built-in plural rules (`_one`, `_other`)
Date formatting with string manipulation	Use `Intl.DateTimeFormat`
No fallback language	Always set `fallbackLng`
Giant single translation file	Split by namespace (common, auth, errors)

Production Checklist

All user-facing strings externalized
Pluralization rules for all supported languages
Date/number formatting with Intl API
RTL layout support (if applicable)
Language detection (browser, user preference)
Translation workflow with translators
Missing key detection in CI

i18n

Internationalization (i18n)

i18next (React — recommended)