Better Auth Localization Plugin

A localization plugin for Better Auth that automatically translates error messages.

Features

• 🌍 Multi-language support - Built-in translations with easy custom locale addition
• 🔄 Automatic translation - Seamlessly translates error messages without changing your application logic
• 🎯 Type-safe - Full TypeScript support with autocomplete for custom translations
• 🛡️ Fallback system - Fallback to default messages when translations are missing
• 🔧 Flexible locale detection - Support for custom locale resolution from headers, cookies, or databases
• ⚡ Zero runtime overhead - Translations are bundled at build time

Installation

npm install better-auth-localization
# or
yarn add better-auth-localization
# or
pnpm add better-auth-localization

Quick Start

Basic Usage with Built-in Locale

import { betterAuth } from "better-auth";
import { localization } from "better-auth-localization";

export const auth = betterAuth({
  // ... your config
  plugins: [
    localization({
      defaultLocale: "pt-BR", // Use built-in Portuguese translations
      fallbackLocale: "default" // Fallback to English
    })
  ]
});

Dynamic Locale Detection

localization({
  defaultLocale: "pt-BR",
  fallbackLocale: "default",
  getLocale: async (request) => {
    try {
      // Use your existing locale detection logic
      // This could come from cookies, database, JWT tokens, etc.
      const userLocale = await getUserLocale(request);
      
      return userLocale || 'default';
    } catch (error) {
      console.warn('Error detecting locale:', error);
      return 'default'; // Safe fallback
    }
  }
})

// Example getUserLocale implementation (adapt to your needs)
async function getUserLocale(request: Request): Promise<string | null> {
  // Could check user preferences from database, cookies, headers, etc.
  // return await db.user.getLocale(userId);
  // return getCookieValue(request, 'locale');
  // return request.headers.get('x-user-locale');
}

Custom Translations

localization({
  defaultLocale: "fr",
  fallbackLocale: "pt-BR", // Can also use built-in locales as fallback
  translations: {
    "fr": {
      USER_NOT_FOUND: "Utilisateur non trouvé",
      INVALID_PASSWORD: "Mot de passe invalide",
      INVALID_EMAIL: "Email invalide",
      SESSION_EXPIRED: "Session expirée"
    },
    "es": {
      USER_NOT_FOUND: "Usuario no encontrado",
      INVALID_PASSWORD: "Contraseña inválida",
      INVALID_EMAIL: "Email inválido",
      SESSION_EXPIRED: "Sesión expirada"
    }
  },
});

Overriding Built-in Translations

localization({
  defaultLocale: "pt-BR",
  translations: {
    "pt-BR": {
      // Override specific messages
      USER_NOT_FOUND: "Usuário não foi encontrado no sistema",
      INVALID_PASSWORD: "A senha fornecida está incorreta"
    }
  }
});

Language Support Status

Currently supported languages:

• 🇧🇷 Portuguese (pt-BR) - ✅ Complete
• 🇵🇹 Portuguese (pt-PT) - ✅ Complete
• 🇪🇸 Spanish (es-ES) - ✅ Complete
• 🇫🇷 French (fr-FR) - ✅ Complete
• 🇵🇱 Polish (pl-PL) - ✅ Complete (Thanks @stripsior)
• 🇮🇩 Indonesian (id-ID) - ✅ Complete (Thanks @finly)
• 🇯🇵 Japanese (ja-JP) - ✅ Complete (Thanks @HRTK92)
• 🇸🇦 Arabic (ar-SA) - ✅ Complete (Thanks @mosaleh-dev)
• 🇬🇷 Greek (el-GR) - ✅ Complete (Thanks @DomVournias)
• 🇸🇪 Swedish (sv-SE) - ✅ Complete (Thanks @yamanadamnor)
• 🇮🇹 Italian (it-IT) - ✅ Complete (Thanks @mattiamalonni)
• 🇩🇪 German (de-DE) - ✅ Complete (Thanks @NiklasDah)
  • 🇩🇪 German (informal, "du") (de-DE-informal) (default)
  • 🇩🇪 German (formal, "Sie") (de-DE-formal)
• 🇨🇳 Traditional Chinese (zh-Hant) - ✅ Complete (Thanks @MarkLee425)
• 🇨🇳 Simplified Chinese (zh-Hans) - ✅ Complete (Thanks @MarkLee425)
• 🇰🇷 Korean (ko-KR) - ✅ Complete (Thanks @MarkLee425)
• 🇮🇳 Hindi (hi-HI) - ✅ Complete (Thanks @MarkLee425)
• 🇹🇷 Turkish (tr-TR) - ✅ Complete (Thanks @furkanczay)
• 🇳🇱 Dutch (nl-NL) - ✅ Complete (Thanks @InvixGG)
  • 🇳🇱 Dutch (informal, "je") (nl-NL-informal) (default)
  • 🇳🇱 Dutch (formal, "u") (nl-NL-formal)
• 🇮🇷 Persian/Farsi (fa-IR) - ✅ Complete (Thanks @Yasser5711)
• 🇷🇺 Russian (ru-RU) - ✅ Complete (Thanks @draneone)
• 🇮🇳 Marathi (mr-MR) - ✅ Complete (Thanks @OutOfBoundCats)

Built-in Translations

The plugin comes with built-in translations for all Better Auth error codes:

• User-related errors (USER_NOT_FOUND, USER_ALREADY_EXISTS, etc.)
• Session errors (SESSION_EXPIRED, FAILED_TO_CREATE_SESSION, etc.)
• Authentication errors (INVALID_PASSWORD, INVALID_EMAIL, etc.)
• Password validation (PASSWORD_TOO_SHORT, PASSWORD_TOO_LONG)
• Social authentication errors
• Account management errors

For a complete and specific list, refer to the Better Auth documentation.

Contributing

We welcome and appreciate contributions! Help us expand language support by adding new translations.

Adding a new language (no merge conflicts)

This repo auto-generates the translations index to avoid PR conflicts when multiple languages are added in parallel.

1. Create a new file in src/translations/, for example nl.ts.
2. Export your translation object and a LOCALES map from that file:

   import { ErrorCodesType } from "../types";
   
   export const NL_NL = { /* ...all error codes... */ } satisfies ErrorCodesType;
   
   export const LOCALES = {
     "nl-NL": NL_NL,
   } as const;

3. Do not edit or commit src/translations/index.ts.
4. Submit your pull request. The index is generated automatically by the build and tests.

The build runs the generator automatically to ensure the index stays up to date.

License

MIT © Marcel Losso Forte