When Caine and Poitier hide the car in the African village, Poitier thanks the headman by saying "Asante sana". This is Swahili, widely spoken in East Africa, but not in South Africa.
I don't know what "Thank you" is in Xhosa or Zulu or Sesotho, or any other of the many official languages (11, I believe), in South Africa. Probably Sidney decided he couldn't get his tongue round those famous clicks. Or "Asante sana" is just more memorable.