languages – Multilingual spell check

I have found a few similar questions but no answers yet.

I use the Microsoft SwiftKey keyboard so I can type in both English and Dutch simultaneously (without having to switch between them). This works great but the system spell checker still put red lines under all my words. I have set the spell checker to ‘use system languages’ but it only uses the first one in the list. If I select manually, I can only use one language. Is there a way to get multilingual spell check?