The translation feature is based on the Bergamot project to provide users with a privacy-aware translation engine where the translation is done locally using machine learning, it’s never sent to a third party, and it’s optimized for consumer hardware.
They have a Firefox translations extension which is pretty good and entirely client side. Unfortunately it didn't support many languages and didn't support Chinese or Japanese so it was a deal breaker for me.
FYI, the translation feature is available in the current stable version (v117), but it's disabled by default. To enable it, go to about:config and set browser.translations.enable to true. I tried it earlier on a German website and it worked well.
This is cool, but beyond anything I wish the linux version of firefox used my default filepicker. It keeps choosing to use the gtk file picker instead of thunar or whatever else I choose.
I hate that translate thing. Blocking languages for translation doesn't even work through UI. It looks so unfinished. And meanwhile FF keeps removing functionality that I do use.