It might sound unbelievable but if you write in multiple languages and mix languages in the same message or sentence, often spell check doesn't work properly. Which is only normal.

I regularly send messages in 4 different languages (living in a bilingual city + frequent use of English and lots of Spanish friends). Sometimes even using 3 languages in one sentence.

Whatsapp kind of improved it now in that you can "activate" two languages at the same time. Apart from that I'm not sure there's much else that can be done.

It's not even that much of an edge case. Brussels is the one of the most international cities in the world, street names exist in 2 languages, a lot of slang and expressions get borrowed from other languages.