(I use Gboard with English and Korean and have to switch.)
Edit: there’s also a button to force the language if I want to override it. It also seems to read the text on screen or remember what apps use what language as it will auto-select the right language most of the time.
It also means things like you get all the diacritics from all of the languages when you long press.
I was studying French, while also working with several Polish people, so I set my keyboard to simultaneously English, French, and Polish, and then I was able to do French lessons without switching, and also able to write my Polish coworkers names easily.
Apparently latest version of iOS keyboard allows for this kind of language mix ( heard about it second hand from an iOS user)
But I agree with the main point, will wait for FUTO Keyboard to support multiple simultaneous languages before I try it.
I switched from GBoard because it's predictions for Arabic were horrendous, made swipe typing unusable.