Now you may send link on this post to your manager, to justify a proper decision

You can make a language request in a issues of repository https://github.com/mozilla/firefox-translations-models/

You’re right, resources are limited and a new languages appears not so fast as we wants. Maybe Chinese developers will collaborate and release their own models for a Bergamot. Work with community are important to achieve the goal when we want to unify translation.

If you really needs to be private for now, you can try to deploy your own offline translation server locally (e.g. with a https://github.com/LibreTranslate/LibreTranslate) and use this translator with Linguist as a custom translator.