I’m on iPhone, using English and Dutch with the built in keyboard and it’s pretty seamless. Basically the layout doesn’t change (it’s always qwerty) but it will detect the language I’m typing and spell check for that language. That might only be possible/easy for languages with a (mostly) common alphabet.

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.