Why wasn’t the dominate language English seeing the hundreds of years of British rule, and why does it seem like all the far right today in South Africa today still Afrikaans?
So to examine Apartheid, we actually have to look a little before the British rule of South Africa. Before the British were in charge of South Africa, the Dutch were actually in control of the area as early as 1652. The Dutch settlers that lived there, lived there for generations. They became as naturalized to South Africa as Americans are in the States today. They uprooted the native population and became natives of a sort themselves.
Control of the area flipped a few times between the Dutch and the British in the early nineteenth century with the British ultimately gaining control. Angered by the encroachment of the British on what they saw as their native land, the Dutch (who I will refer to as the Boers or Afrikaners) began moving north into the interior. In 1835, we see the largest of these migrations in the Great Trek and the formation of the Orange Free State and Natal. Ten thousand Boers migrated as a sort of displacement. All the while, tensions are obviously high with the actual natives. This resulted in establishing a nationalistic identity known as the "Afrikaners" based on extreme racial segregation. Afrikaans was a form of Dutch that they spoke.
The Boer states were attempting to claim independence from British rule, and in 1899, the Anglo-Boer War broke out. This furthered tensions in the area but brought the Dutch settlers back under British rule. While the British beat the Afrikaners, they did not wipe out the nationalism. They bolstered it.
Despite the British finally ruling the area, the Dutch had been installed for 250 years. They were in control of the government, much of the land, and much of the wealth. The British may have had a loose kind of political control in the area, but the hard political power was in the hands of the Afrikaners.
I think of it as a similar concept to the "brown Englishmen," except a white version. The Afrikaners were the natives that dominated much of the political and social landscape. When the National Party (NP) came into power in 1915, they were Afrikaners not British nor Dutch. It was the language they spoke, the language of the party, and a language that could be used to keep racial segregation in place. The British allowed them to keep their power as long as that power ultimately served the British.