No rules? Correct. But even a basic level of realism dictates that characters from other planes of existence have a different accent. I mean, in real life accents vary wildly over short geographical distances. It really does break the game for me personally, making it very difficult to get into the story and try to think of this as a real place with real people. It's just, 'Yeah, this is a game with a made up story.' That's fine for a Mortal Kombat story mode, but for BG I expect more.
But no matter what, the accents given to these different people will be arbitrary. Look at Mass Effect--You have humans that speak in different accents and then the aliens have some arbitrary accent assigned to them. Turians speak with American accents. The Quarians speak with several different accents. I think the Krogans also sounded American, if I recall. But they were all alien--shouldn't logic dictate that they speak with accents completely unknown to us? But that's hard to pull off and more likely to come across as silly so they're given different earth accents. One could argue that if they're using a universal translator or something then they should all speak with the same accent as the listener for clarity's sake. (Which also begs the question, depending on the setting, if you want to look at things realistically, why are they speaking the same language at all?)
Realism can only be applied to fantasy arbitrarily because it is *fantasy*. Ergo, execution comes down to personal preference. I like the the British accents and find a lot of variety present in them. You don't. Both perspectives are fine.
