It's no so much that the person is supposed to look "unintelligent," but more that the author is trying to capture a particular dialect of English, a particular colloquial way of speaking. If you visit certain parts of England, where Harry Potter takes place, you'll find people who speak like this. Have you seen any of the Harry Potter films? They would give you a better since of how this kind of speech actually sounds. I can't comment on the Russian translation, but it's probably rendered as grammatically correct because there's no way to translate this specific dialectical way of speaking into Russian. Hope that helps!

You are right regarding J K Rowling's intentions, and about the fact that there are people who use non-standard grammar - ain't, double negatives, he don't, and so on - throughout the English-speaking world. It's also a common device for authors to use this non-standard grammar in dialogues to show that a particular speaker is ill-educated or of a lower social class. Charles Dickens, for example, was famous for using this device. In fact, in this quote, the non-standard features are more phonological than grammatical - the dropped 'h' and the lazy dropping of the 'have'/'ve' is mainly suggestive of the way that this person sounds. As for why this line was translated into standard Russian, I have no idea. Does Russian literature not use this convention? Is there no tradition of putting poor grammar into the mouths of uneducated Russian peasants when they take part in dialogues in Russian books?