From a quick state of the art about internationalization on npmjs.org, I would recommend LinguiJS. Because:
it can be used both in React and plain JavaScript,
it uses real phrases in any natural language can be used as the pivot language instead of formal keys (making source code more readable for coders, and translation more contextual for translators),
it provides a client to automate the internationalization step,
From a quick state of the art about internationalization on npmjs.org, I would recommend LinguiJS. Because: