Personally I think that J.K. Rowling done a great job with people's accents throughout the books. Take Fleur for example, you can tell that she is French but her accent isn't to heavy so you can understand her. Now often times in fanfics people put on Fleur's accent far too thickly! They do this with Viktor too and it's very difficult to understand. I don't understand why people do this it's really hard to understand and I imagine that some people would be offended by that. In the books Fleur's accent is obvious but it doesn't make it difficult to understand what she's saying. I've seen fanfictions like this:
"'Ello Veektor 'r yoo reddy too poot yoor nam een zee Gobleet ov Fyre?" Fleur asked
"Yaz Flur I eez reddy leetz doo sees,"
That is not how French and Eastern European people speak! Please keep your accents to minimum so as not to offend people and so that your readers can understand what you're saying.