Hi @gil ,
Funny that you ask this now. I just signed up to this board to post exactly about this.. perfect timing!
I just developed a new plugin (see here) to do exactly what you mention: in-app translation of emails. It is not perfect, and definitely not tested thoroughly against all Linux flavours and versions (the packaged deb is for Ubuntu Noble), but it works for me.
For now it has options to run completely local (less accurate but a bit faster if you have a GPU) or online (sends the body text to Google.. Somewhat slower for longer emails but more accurate results). I plan to add more online providers in the future, and possibly the option to hook local LLMs.
Let me know if you end up testing it! I am curious to know if it works for others too 
Cheers,
Andrea