i'm bilingual too, i just deactivated autocorrect. it's too frustrating
Even mixing languages works quite well in my experience. I use "Gboard" which I think is the default keyboard app? simply with all three languages enabled, and I don't switch keyboard (using always the same disposition, the one of my native language which isn't the language I type most on my phone).
IPhone is unusable in this regard. Switching from one language to the other is just not cutting it. That's simply not how polyglot people communicate.
I can kind of understand why, but I don't think it would be impossible to design a way for it to work.
So for example, you can enable it for English and Spanish, but not for English and Russian. In fact, it doesn't even seem to be supported for different languages that use Cyrillic.