transloco

Transloco is an internationalization (i18n) library: it handles translating your UI text into different languages at runtime. It's the translation layer between your app and your users' preferred language.

The core library is free and open source under MIT. You get lazy-loaded translations (only load the language the user needs), runtime language switching, rich template syntax, SSR support, and a plugin system. It's the most popular i18n library in the Angular ecosystem outside of Angular's built-in i18n.

Transloco has some paid plugins: the Keys Manager ($50 one-time) auto-extracts translation keys from your code, and the Optimize plugin ($50 one-time) removes unused translations at build time. There's also Transloco Locale ($50 one-time) for number/date formatting. Angular developers: this is the library to use for i18n. The free core does everything you need. The paid plugins save time but aren't required.

The catch: Angular only. If you're using React or Vue, look at i18next or js-lingui. The paid plugins are nice-to-have productivity tools, not essential features; you can manage keys manually and optimize bundles yourself. Also, the documentation assumes you know Angular well.