It's a bit tricky. My phone's language is English, but I have many keyboards. When the keyboard is set to English, I can also type in Spanish or English, but none of the other keyboards I have. In fact, when I start to type in one of the other languages when the keyboard is set to English, it seems to assume it's Spanish and the options it gives me are all in Spanish.
When the keyboard is set to Spanish I can, predictably, type without issue in both Spanish and English, but not Italian or French or any of the other languages.
When the keyboard is set to Italian I can type in Italian or English, but I can't type in Spanish, etc.
So it seems that whatever the language your phone is in is the one that will be able to be typed in regardless of the keyboard language. I'm not sure why Spanish is the one I'm able to type in when the keyboard is in English, but maybe because my region is US and Spanish is the keyboard I use as much as or more than English.
I'd love to be able to just type and not have to worry about the keyboard, but oh well.
I do have a complaint, though. The Spanish predictions and corrections are absurd since the update. For example, if I accidentally type "cisas" instead of "cosas," it leaves it as "cisas" and even replaces the correct word.