To change the primary language on Facebook.com is quite simply and is for everyone easy accessible.
Login on facebook.com click the button for settings (... see Image-1 Arrow-1) , and select the text "Account Settings" (... see Image-1 Arrow-2) . In the "General Account Settings" click "Edit" Language options. (... see Image-1 Arrow-3).
In Language setting, change you primary language! change the language (... see Image-2 Arrow-1) and "Save Changes" (... see Image-2 Arrow-2)!
FAQ 7: Updated on: March 3th 2018 21:15