The lyrics to "Nkosi Sikelel' iAfrica" are written in the five most widely spoken of the country's eleven official languages: Xhosa, Zulu, Sesotho, Afrikaans, and English. Prior to South Africa's first democratic elections in 1994, the country had two anthems. The first, "Die Stem van Suid-Afrika" (The Call of South Africa), had been the country's official anthem since 1957.
The other tune was "Nkosi Sikelel' iAfrica," a former church hymn. This song was sung as an unofficial anthem of the people in protest and resistance to the apartheid government of the time. Soon after being elected, President Nelson Mandela named the tunes as co-anthems. In 1997, he had the songs adapted into one anthem as a gesture symbolic of a newfound national unity.
You can read the entire anthem (in English!) and listen to it here.