Spell check for more than 1 languages in Android

I try to make the spell check work for more than one language on my Pixel 2 (Android 8.1) with Gboard. My question is *not* about auto-correction or word prediction. The spell check is activated and the two languages (UK and German) are activated in Language preferences and Gboard preferences. Changing the language while typing via the space bar does not solve the problem. So far, it is only working in Google Docs: I read the answers on [Stack Overflow - Adding a second dictionary to the spell-check in Android 4.3?](https://stackoverflow.com/questions/20197421/adding-a-second-dictionary-to-the-spell-check-in-android-4-3 ), but they did not solve the problem. I guess a solution for SwiftKey is even more difficult?