From my experience, when asked D'ou venez vouz? or tu viens d'ou?. people, both native french speakers and those who have learnt french as a second language, will answer either way. Either, Je viens d______________(country name). or je suis______________(nationality). No one pays much attention to whether you have answered in accordance with the structure of the question, only that you reveal the requested info, ie what part of the earth you call home..