[luvbach1]

Would somebody from South Africa living in Brazil be a South African-South American?

I think you were being facetious; but no, because there is no nation called South America but there is a nation called the United States of America (also known as America.)However, the person you envisioned could be called a South African-Brazilian.

[Cronos]

As another poster pointed out, recent migrants from Africa know their origin, so someone from SA would know if they are Xhosa or Zulu etc.

[GOPJ]

There no ‘nation’ called Africa either...