I have seen various cases where the default language of Facebook gets changed automatically. Most users are reporting that their language has been changed to Arabic, Polish, French, Vietnamese, etc without any reason. Go here and read all the comments to get an idea what I am talking about.
The most popular way to change the language is to go at the bottom of the page and then change it by clicking the Language link. But this method is reportedly not working for most users, when they change the language using this method, it gets changed back to some other language.
So here is how you can fix it, go to this link:
and simply change the language to English, after about 2 seconds the page will reload automatically and the default language from now on will be English.
I believe this language change took place according to people’s geographic location after Facebook announced new translations. Anyways, the above mentioned link will definitely solve your problem. Enjoy!