It is very hard to make a feature film, because feature films cost a lost of money. People or institutions (like banks) that invest money in feature films want to know that they will be guaranteed returns on their money. Hence the star system. By casting big names investors hope to ensure the film will succeed (i.e. that a lot of people will buy tickets to see it). So often South African films with stories about very South African events have featured actors who have struggled to authentically portray South Africans.
Cry Freedom, about the relationship between Steve Biko and Donald Woods, starred Americans Denzel Washington and Kevin Klein, and featured wince inducing attempts at Afrikaans accents. Athol Fugard's Boesman and Lena had Americans Angelina Basset and Danny Glover in the starring roles, when there were a host of South African actors who could have better played the parts.
Andre Brink's A Dry White Season was made into a film by MGM in 1989. Shot in Zimbabwe, it starred Donald Sutherland and Suzanne Sarandon.
It is interesting to note that most of the anti-apartheid films using foreign actors have been commercial flops.