Okay, thanks. The switching between languages is not perfect yet, but works okay. I regularly get suggestions for another language than I am currently typing.
I just start writing in the different language and the suggestions + autocorrect switch for me. All the languages (other than Korean) use the same keyboard layout and for the umlauts I need to hold the aoeu shortly to get the right umlaut, or I just rely on the autocorrect.
Back to heliboard. I found the setting. In the languages selection, it seems it is only a toggle and to get the menu you need to long press. That needs to be fixed and put the multi language option outside.