I have reservations about making up Japanese names when there is no canonical, however, if you're going to do that, it should be アフガニスタンちゃん or perhaphs アフガンちゃん, not アフガニスタン人ちゃん. Also the corresponding romanisations would be Afghanistan-chan (or Afghan-chan) and Tibet-chan.

(I am not a native speaker, either, but I am fluent.)