wat?
first of all, yeah they do. Even in a fictional anime world people still have ethnicity's. If Global and Misa are talking on the bridge they're probably both speaking the same language, and if that's the case they're going to talk with whatever native accent they have because that's how people talk.
And even if the situation is that they're all speaking different languages and they're just having everyone speak one language for convenience sake, you'd STILL want people to have different accents so the audience watching can go; "Oh, he's got a Russian accent, he must be speaking Russian and we're just hearing English because the creators of the show are know we're too lazy to read subtitles."