Update - some protesters showed up at the auditions (about the casting). These casting people are clueless, and I quote from the head of casting herself, advising extras "If you're Korean, wear a kimono, if you're from Belgium wear lederhosen, if you're from Africa wear a scarf so you look like you're a Ukrainian viliager."
Anyway, they have found out that the nations have been divided up by race as well as elements so:
Water Tribe: White
Air Nomads: White
Earth Kingdom: East Asian (Korean, Chinese etc.)
Fire Nation: South Asian (eg, Indians, Pakistainis etc.)
So Toph, will be the only character who looks like what she's supposed to be.