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.