Have you tried Heliboard? You’ll need to download a (proprietary?) library for it though. https://github.com/Helium314/HeliBoard

+1 for Mailbox.org! Works well with FairEmail (Android) and Evolution (Fedora). Evolution is a bit more polished than Thunderbird in my opinion.

And looks like the web interface gets a revamp this year: https://mailbox.org/en/post/we-are-celebrating-10-years-of-mailbox-org

It has that! 😃 Long press a language in de settings, a pop up menu opens, and add a second language there.