Oh? Thanks Heidrun, I think "você" actually means "tu", so I'll edit the French version. Thanks a lot!
(and to answer your question, as it readed before I edit, no one could tell exactly because of the lack of context; to know, one would have to see wether this question is asked to one or several persons; eg : polite "vous" : "madame, je voudrais savoir en quelle langue vous vous exprimez correctement"...or plural vous : "Mesdames, je voudrais savoir en quelle langue vous vous exprimez correctement"...)
Oh, I didn't expect to reveal a mistake by just asking that simple question But now it's clear to me, why the translator into German used "you" (sing.)