@andrewwade (dropped Casey). You make 2 points. That Afrikaans, Xhosa, and Zulu aren't supported is very logical — they are wwwaaaayyyyy too far down the list if you were to order world languages in order of localization importance. which correlates to how many speak it as their first language. With many dozens of other languages ahead of them that are more important (i.e. more widely spoken) to get to first.
But even more importantly, that for Apple specifically, those 3 languages can be moved even (much) further down the priority list given the iOS/Android split in South Africa (80%+ Android).
The other point about "old" languages is separate and has its own reasons, which one can definitely have opinions on, but it doesn't really affect the prioritization considerations above.
(And to head off any drive-by replies accusing me of being a typical ignorant American, I grew up speaking Afrikaans in South Africa, so I feel I have a right to opinions about it.)