Sotho and Xhosa are both Bantu languages and, since 1994, official languages of South Africa. Sotho, as well as being the official language of Lesotho, is spoken in the Free State, southern Gauteng and to a lesser extent in the area surrounding Pretoria. Xhosa is the most widely distributed language in South Africa and is spoken in the Eastern Cape, the area where Lucy's farm is situated. Watch the video below, for a short demonstration of the Xhosa langauge and the click sounds which make it unique.
In Disgrace, the football commentary alternates between the two tongues, as they are the primary languages spoken in the respective areas the teams represent.
The fact that David and Petrus are watching a match with commentary in Xhosa and Sotho, reflects the fact we are in post apartheid South Africa. Pre 1994, Afrikaans and English were the prime languages heard on television, and only after the ANC came to power was there a move make the TV channels reflect the languages actually being spoken.
Another important change, as far as language was concerned, came with the introduction of a new South African national anthem. Nkosi Sikelel' iAfrika (God Bless Africa in Xhosa), for many a years a song of protest and a symbol of defiance against apartheid, was incorporated into this anthem.