Write an article based on this "Open Settings. Tap General. Tap Language & Region. Tap iPhone Language. Tap on a language. Tap Done. Tap Change to [selected language]."
It's the app with grey gears that can be found on your Home screen.  You may have to scroll down to find this option. A list of languages will appear. A blue check mark will appear next to it. A confirmation screen will appear. Your device's interface, including Facebook, will now be shown in your selected language.