I wrote back in 2016 a guide to help if your language choice got changed on Facebook. It was supposed to be more of a visual guide because you wouldn't understand the words to navigate through the menus to get back to your language of choice.
read here: davescomputertips.com/how-to-change-facebook-language-back-to-english
I don't know if it's still a problem but Facebook has gone through some noticeable changes, so here is an updated version. So sign into your account.
Then click on Settings & Privacy. It's got the gear symbol beside it (or flower).
Then click on the Language option, last item on menu. It currently has a globe symbol beside it.
This will take you to Language and Region Settings.
Click on the word Edit (in English).
Then in the Facebook language section, click on your language choice in the drop-down menu.
Then click on the first blue button to Save Changes.
So, has anybody had a language change issue since the new look came in?