Recall that the solution to having your autocorrect mess with your French or Spanish typing has been to add more keyboards under Settings/General/Keyboards or //Language & Region.
Now your iPhone will allow you to make one keyboard bilingual -- new choice shows up and you just have to check it. I'm trying out two bilingual keyboards -- one English (UK) + French and the other English (US) + Spanish (Latin America). Here's hoping I will have to do less correcting of the autocorrect :-)