On 'answers.microsoft.com' there's an old discussion called "Outlook 2016 Spelling check in Subject line different from body text", the discussion is closed, but the error still exists in Outlook (I'm running Version 2006 (Build 13001.20384)) on Windows 10, everything updated.
I know MS is busy!!!!!!!!, but fixing an error that is 3-4 years old, should have been done by now :-).
The error is, that the subject line is checked in English and the body in my language (Danish). Looking around, I see that many other users have the same problem in many other languages, so it's not related to any specific language.
As you see, the Danish word is checked in English in the subject line and in Danish in the body.
I have read many posts about this, tried to change Region, local languages, the order of which they are used in Outlook etc., still not fixed.