Thanks Paul for the answer. When I wrote switching to the main current language I meant to check in which language the mail is written. Not the language of the program nor the language of the system nor the current keyboard layout should matter. I (and I think this is true for many / most people) use one keyboard layout, regardles which language I am using.
If I write an e-mail in english or german or dutch the spell checker should look up the word in the three specified dictioniaries. A second option (not as good) would be to determine in which (main) language the e-mail is written. This is pretty straight forward to implement. After a few words a program can tell which language is used by just looking up the words in the dictionaries. If that is too expensive (computing wise) one could use a stripped down “check dictionary” which includes the top 100 used words of the different languages and just count which dictionary fits best. This would seldom fail, if ever.
Regards
Peter