i18next: learn once - translate everywhere 
i18next is a very popular internationalization framework for browser or any other javascript environment (eg. Node.js, Deno).
i18next provides:
- Flexible connection to backend (loading translations via xhr, ...)
- Optional caching, user language detection, ...
- Proper pluralizations
- Translation context
- Nesting, Variable replacement
- Flexibility: Use it everywhere
- Extensibility: eg. sprintf
- ...
For more information visit the website:
Our focus is providing the core to building a booming ecosystem. Independent of the building blocks you choose, be it react, angular or even good old jquery proper translation capabilities are just one step away.
Documentation
The general i18next documentation is published on www.i18next.com and PR changes can be supplied here.
The react specific documentation is published on react.i18next.com and PR changes can be supplied here.
Gold Sponsors
From the creators of i18next: localization as a service - locize.com
A translation management system built around the i18next ecosystem - locize.com.
With using locize you directly support the future of i18next.