Bitwarden is open source (server, plugin and app) and can be self-hosted so it’s not centralised in any way that matters.
Also, I think an honest freemium offering is the best way to do it - have those that are willing/able to pay subsidise those who aren’t. It doesn’t have to be a slippery slope, and that’s not exactly common in the open-source world. After all, you can just fork it and go your own way if you’re not happy. Also, running servers isn’t free, and being able to remunerate the devs a little is no small thing.
So, in summary, use Bitwarden. You can set up your own server and install the plugin/app yourself if you want.
As noted, this is an old article. You can install the plugin here: https://addons.mozilla.org/en-GB/firefox/addon/firefox-translations/
I just tried it on https://www.tagesschau.de/wirtschaft/technologie and it’s definitely good enough to be usable, although it has translated the top story as “What the new data glasses from Apple can”. Google Translate’s version is almost the same for most of it, although it gets “can do” right.
It initially recognised that it could translate feddit.de but seems to have stopped now. Hmm.
Anyway, even though German->English is a pretty easy test given that English is a Germanic language, I’m happy to leave it installed and test it in the wild.