react-i18next

React-i18next is the most widely used internationalization library for React. It wraps i18next, a battle-tested translation framework, and gives you React hooks and components to swap languages without reloading the page.

MIT license, JavaScript. The core hook is useTranslation: call it in any component and you get a t function that returns the right string for the current language. Supports namespaces (so you can split translations by feature), interpolation, pluralization, context-based translations, and lazy loading of language files.

Fully free. No paid tier, no premium features. i18next the ecosystem includes optional paid services (locize for translation management), but the library itself is completely open source. Works for every team size. Solo developers building a multilingual side project to enterprise teams managing 40+ languages, the same library handles both. The setup is lightweight: one provider component, JSON translation files, done.

The catch: translation file management becomes painful at scale. When you have 30 languages and 2,000 keys, keeping JSON files in sync is a real job. That's where paid tools like locize or Tolgee help, but they're optional. Also, the i18next configuration object has a LOT of options, and the docs assume you know what you want. First-time setup can feel overwhelming.