Only way to do this is probably with a pre-1900 POD like others have said without going into ASB territory. I think the best bet would be for France to officially make the Cape a dumping ground for the Huguenots, which means that at least around 500,000 people are sent to the Cape sometime in the late 16th or early 17th century (also, we would probably get a South African nobility due to many Huguenots being nobles themselves, so that's always cool). I thought
this timeline was pretty good at exploring that scenario.
And if the Huguenots have somewhere definitive to go where they can legally practice their religion that is also French-Speaking, then it would probably attract immigration from the 13 colonies and other European states. And somewhere between 500,000 to 1,000,000 French protestants in the early 17th century is a HUGE population to start with, especially because of the higher than average birth rates. By sheer numbers alone they would probably dominate the Cape and what would become Natal and the Free state. That is of course not even counting immigration from other countries. Simply by the fact that the Cape is so out of the way, very Protestant, and would be largely left to its own devices would make it an attractive place for settlement. If we get the British to control it in the 18th century, then it is likely that we get at least a few loyalists to settle in the Cape. It may or may not become English-speaking, but when self-government is granted (probably much earlier than OTL) we'd probably see something like the Cape Liberal tradition take hold across much of the country, without the fear of Black African domination. Of course the Zulus and Xhosa could have a much tougher time ITL, being swamped by so many whites that they either accede to be incorporated into South Africa or migrate further North, which would have quite a few interesting effects on native African tribes. And if what is OTL Free State and Transvaal is not already majority or plurality white by the mid-19th century, the gold and diamond rushes will probably make it so. This would make South Africa Britain's most important settler colony by a large margin, and would further siphon away immigration from Australia, New Zealand and Canada.
As for post-1900 effects, having a different colonizer for South Africa might not create too many butterflies simply because the Cape is just so far removed from everything else in Europe, but who knows. For sure we'd have a much more liberal South Africa than OTL.