What I really want is a swipe model that can tune to multiple languages simultaneously when a keyboard layout can support it (English + German, German + French, etc). I rarely need more than 2 languages simultaneously, but the "switching between 2 languages" point is painful.

Trilingual here. Gboard on my pixel 10 pro does support swipe type while code switching between multiple languages. Works reasonably well. Accuracy is a bit lower for words that are collisions across languages

What is "code switching"?

When you're multilingual, you'd switch between languages on the fly, mid-sentence even, just because the words in other language come to your mind quicker, seem to better match nuances, etc.

This would be cool. I suspect that people don't implement this model because it's a long tail of users requesting this. This is totally feasible technically.

Yes, this is a showstopper for me. Preciso escrever in several languages.