Not sure why you're mentioning Mozilla here, the only thing in common with the submission is that it happens to be hosted under mozilla.org. The ECMAScript Internationalization API seems to have been written and contributed to by mostly non-mozillians, none of the contributors listed at https://webplatform.github.io/docs/apis/internationalization... are related to Mozilla as far as I can tell.
Project Fluent is a great recommendation though, haven't seen before but looks really useful.
Project Fluent is a great recommendation though, haven't seen before but looks really useful.