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.
If you think a Object Oriented Programming is a framework, then you can think about BEM like a framework too. But a correct word is a methodology. A "framework" have too generic meaning.
Methodology gives us a methods how to improve software quality. This methods like an interfaces in programming that we must implement as programmers