Soweto - Capetown ?

This movie is a movie adaptation of the drama "Soweto Burning", and most posters and comments repeat this.

But nobody mentions that the movie was made in Capetown, more than thousand kilometers away from Soweto (which is next to Johannesburg). It is strange, because the author of the play and the movie is the same person.

Also, Soweto is worldwide known for decades of riots against white apartheid government, it is a symbol of the struggle of deprived black majority. So there is no danger that it wouldn't be understood outside RSA.

Or, maybe, Kettle didn't want this struggle to be too emphasized in the movie, so this outside violence wouldn't overshadow the intimate story (and it's one but so tragic moment of violence), so putting his story in less symbolized setting he could have make us concentrate on three characters that are not so determined and limited by place or time. Joseph could have this way be Jew in nazi Germany, Navajo in USA, Aborigine in middle Australia, Palestinian in Israel... But, if so, what's the use of Mandela speech near the end of the movie?

Any idea or (even better) information?