Better Auth Localization Plugin

A localization plugin for Better Auth that automatically translates error messages from the core library and official plugins.

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

The getLocale option lets you dynamically detect the locale for each request.
It receives a raw Request object, so you can read cookies, headers, JWT tokens, database info, whatever makes sense in your app.

This plugin does not know your user/session automatically. You decide how to extract the locale.

Example

localization({
  defaultLocale: "pt-BR",
  fallbackLocale: "default",
  getLocale: async (req) => {
    try {
      const locale = await getUserLocale(req);
      return locale ?? "default";
    } catch (err) {
      console.warn("Error detecting locale:", err);
      return "default";
    }
  }
});

Implementing getUserLocale (choose the strategy you prefer)

Below are a few common ways to detect the locale.
Pick one or combine them depending on your app’s architecture.

1. Read locale directly from a cookie (simplest)

async function getUserLocale(req: Request): Promise<string | null> {
  const cookieHeader = req.headers.get("cookie");
  const cookies = parseCookie(cookieHeader);
  return cookies.locale ?? null;
}

2. Read locale from a custom header

async function getUserLocale(req: Request): Promise<string | null> {
  return req.headers.get("x-user-locale");
}

3. Decode your session/JWT and read the user's preferred locale

Useful if your authentication stores user data inside a token.

async function getUserLocale(req: Request): Promise<string | null> {
  const cookieHeader = req.headers.get("cookie");
  const session = decodeSession(cookieHeader); // your own logic

  return session?.user?.locale ?? null;
}

4. Look up user locale in your database

Only makes sense if your session/cookie includes a userId.

async function getUserLocale(req: Request): Promise<string | null> {
  const cookieHeader = req.headers.get("cookie");
  const session = decodeSession(cookieHeader); // extract userId
  const userId = session?.userId;

  if (!userId) return null;

  const locale = await db.user.getLocale(userId);
  return locale ?? null;
}

If you have suggestions for more examples or want to see a built-in helper, feel free to open an issue!

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)
• 🇩🇰 Danish (da-DK) - ✅ Complete (Thanks @BjornFrancke)
• 🇷🇴 Romanian (ro-RO) - ✅ Complete (Thanks @danielavornic)
• 🇮🇶 Kurdish Sorani (ckb-IQ) - ✅ Complete (Thanks @didar-dev)
• 🇧🇩 Bengali (bn-BD) - ✅ Complete (Thanks @ktisakib)
• 🇦🇿 Azerbaijani (az-AZ) - ✅ Complete (Thanks @mehdiasadli)
• 🇻🇳 Vietnamese (vi-VN) - ✅ Complete (Thanks @trinhvanminh)
• 🇺🇦 Ukrainian (uk-UA) - ✅ Complete (Thanks @tarasromil)
• 🇨🇿 Czech (cs-CZ) - ✅ Complete (Thanks @michaeldoubek)

Built-in Translations

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

Core Library:

• 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

Official Plugins:

• Admin plugin error messages
• Anonymous authentication plugin
• API Key plugin
• Device authorization plugin
• Email OTP plugin
• Generic OAuth plugin
• Have I Been Pwned plugin
• Multi-session plugin
• Organization plugin
• Passkey plugin
• Phone number plugin
• Two-factor authentication plugin
• Username plugin

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. Run the locale creation script:

   npm run generate:locale
   # or
   pnpm generate:locale
   # or
   yarn generate:locale

2. Input your desired locale (e.g., nl_NL, fr_CA, es_MX)
3. The script will automatically:
   • Create the main translation file with the correct structure
   • Generate plugin-specific translation files with proper types
   • Update all necessary LOCALES objects
   • Use the correct error code types for each plugin
4. Translate the generated content to match your locale
5. Do not edit or commit src/translations/index.ts.
6. 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