vmomenv we don't have any official guides at this point. The best is to take a look at existing language packs to see how it's done. It's basically just an extension with some special keys in composer.json, an extend.php file calling a single extender, and a list of translation files organized by extension name. And there's a javascript file to configure the date library that can be copied from Moment.js source code.
A few projects were also started to manage translations via a third-party service. I think one has a public discussion on the forum, but I can't find it right now.
