As a native Korean, the expression '어디 사람이에요?' sounds natural :) This is same as '어느나라 사람이에요?' and 어디서 왔어요?, and my understanding is that '어디 사람이에요?' can be more oftenly used to ask, even to Korean came from different province or city, as this does not include 'country'.

Direct translation of "어디 사람이에요?" to English seems do not make sense. However, in Chinese, we have similar expression(您是哪裡人？) and it's very common, so I think "어디 사람이에요?" is not an error, it's just another style.