This sounds very similar to Firefox Translations https://addons.mozilla.org/en-US/firefox/addon/firefox-translations/
I wonder how they differ? I’ll have to give it a try, FT only has engines for specific languages -> English, maybe this will have more.
You can read blog post about Linguist features https://vitonsky.net/blog/2023/07/13/linguist
Long story short - a linguist is a universal tool for translation, you can use any translator you wants, even your own, that deployed at localhost.
Embedded firefox translator even can’t translate a selected text on a page as i know and uses only Bergamot Translator with limits you mention. Just for context, a Linguist will translate images in next releases https://github.com/translate-tools/linguist/issues/310
Linguist uses an embedded Bergamot translator for offline translation too, so unfortunately language pairs the same as in native firefox translate, but unlike the one, with a Linguist you can pick any other translator when you want, like google translate, ChatGPT or LibreTranslate for case if you needs an offline translation, but with additional languages that Bergamot does not have