Ah ok, I agree and I have some of the same issues. I'm using Firefox and the spellchecker works well. I regularly switch between 3 languages. Same for Thunderbi

Ah ok, I agree and I have some of the same issues. I'm using Firefox and the spellchecker works well. I regularly switch between 3 languages. Same for Thunderbird, plus there is an extension that remembers which language I'm using with a specific contact, which is great. Regarding the en-US locale, I've read that some people use the en-CA locale instead, this way you get (partly) American spelling, but with metric units and international standards like A4 paper size and reasonable date format.