Unfortunately the automatic language selection does not work. I write often emails in German and English or I answer a mail in the same language, but I always must select the language for the spell check.
That is not very helpful.
There is no automatic detection of language for spell-check.
Ok?!
This is what I always assumed with the menu point “Automatische Rechtschreibkorrektur verwenden”.
But it would be a nice feature anyway 
I’d also like to request this feature.
I love em Client and everything about it. But changing the language regularly several times per day is a bit annoying.
I hope you guys can come up with an automatic language detection for the spell check.
Did you try the three dots within the mail composition window → Spellcheck language → Automatic by Language?
I tried that very briefly just now and it seemed to detect the language I was using, but you might be doing something more sophisticated than my test. 
There is a current Sleekplan suggestion for an automatic language detection for spellcheck which you can vote for via the following link.