Step 3 : In the Choose Language dialog box, click the arrow to display a list of languages that Facebook supports.
Click the arrow to select the display language on Facebook
Step 4: Select the language you want to change, here we are using Facebook in English so we will change to Vietnamese.
Select Vietnamese language for Facebook to display in Vietnamese
Step 5: Click Save Changes to save the Facebook language settings you just selected
So, you have successfully changed the language on Facebook. You can change it back or change it to any language you like in the same way.
Good luck!