I suggest that you learn Steven Pinker's book on the language instinct regarding this phenomenon.
The first issue that I have is that you claim that SA has 11 distinct official languages. This is actually wrong and linguists have pointed this out numerous times. The 11 languages comes from the Apartheid regime's tribal classifications.
For all practical purposes, Sepedi, Tswana and Sotho are the same language. The differences between them is what we would call dialects or variants in most other languages.
The same case can be made for Zulu, Ndebele, Swasi and Xhosa.
Venda is the only one that seems to stand out a bit, the reason being that the Venda people were traditionally isolated.
Languages follow a bell curve of distribution and if you read Noam Chomskys work on this subject you will realize the massive role that the state played in trying to enforce a certain dialect onto populations. This however never really works and kids often revert back to speaking the dialect of their peers.
The second issue is that you make the assumption that culture is equal to language. This is also not true. Children for example have a culture of their own that is often distinct from their parents. Also geography plays a big role over here. An English white kid in the North of SA has a culture most closer to an Afrikaans white kid than between Afrikaners of the North and South of the country. This is why English folk in Pretoria share many conservative views of the Afrikaners alongside them.
The next issue that we need to clarify is that of 'mother tongue'. It is true that babies at a young age respond to the frequencies of their mothers, however children are more likely to speak the dialect of their friends. The concept is something between maatjietaal and moedertaal. This is why an Afrikaans kid who goes to an English school will not show any big development issues (math and science is often an exception), but a black class in SA who is forced to speak English when none of their friends do have a tougher time struggling. Hence the case for teaching in mother tongue actually makes sense.