There are a few things to check. First, do you have all your preferred languages registered on your phone? Go so Settings | General | Language & Region and check that all the languages you type in are listed in the box. If not add the missing one(s).
Next, decide if you want your iPhone to correct your typing at all. If you are like me, your literacy is better than Apple's crowdsourced auto-correct feature. I got so frustrated with my iPhone changing my correct words to incorrect ones that I went to Settings | General | Keyboards and turned off both "Auto-Correction" and "Predictive".
One or both of these suggestions may make your life easier. Good luck.