I don’t know if the mixed autocorrect is a new feature but if I switch to Hungárián thai is WhatsApp autó correct Ford tó my Message. French is similarité screwed up although after a few words it does seem to offer English autocorrect options. Maybe it depends on language?

For me (writing in mixed English and Spanish) it's never necessary to actually change the keyboard unless I just want the ñ key available. I wrote half of this with the English keyboard and half with the Spanish keyboard and it didn't make any difference with respect to autocorrect. If I want to use Spanish text in the middle of my sentence por ejemplo así, it just works. It autocorrects "asi" to "así" without switching to the Spanish keyboard.

It could be a function of time spent writing bilingual text. If I add a new keyboard (I tried Hungarian as well) and it didn't work for that. Or it may only work with exactly two languages? It's not a documented feature as far as I know, so it's hard to say what the conditions and parameters are.

For good measure I also tried reseting my keyboard dictionary just in case what I'm describing is just the effect of intermingling my autocorrect dictionary with so much Spanish over the years that it can code switch now, but nothing changed.

Better - and necessary for my use case - is the first word. I just type whatever I want and it suggests/tries both languages at the same time. Few words is too late since I want to use 1-3 words in the other language only.

That's generally exactly how it works for me. I don't think I've done anything special with my configuration apart from enabling two different language keyboards.