You're right. In the Arabic version of the books, this isn't the only "problem". A lot of names get lost in translation, and if I hadn't read the English versions of the books, I would have get lost as well. I guess the translator doesn't really care about the novel as events and relationships between characters as much as they care about getting the book translated and doing their job, I know that this is annoying for us potterheads, but since we know the difference, does it really matter if it's "vous" or "tu"?