Use nl translations when no af available
While Dutch and Afrikaans are somewhat mutually intelligible, I strongly suspect that most Afrikaans readers would be pretty surprised by Dutch, and pretty unfazed with English. When it comes to computer/IT terms, Dutch and Afrikaans are pretty different. Between the two alternatives, English is very commonly used (and at times forced into our lives), so many people are familiar with it. Many are not even aware that technology can be anything but English. Half-translated user interfaces for software is fairly common, and the fallback is usually English. Windows and Firefox can only fall back to English, for example.
A typical adult in South Africa or Namibia has studied English for 12 years at school, and might have read one prescribe book in Dutch. Most people don't get much exposure, unfortunately.
Does mediawiki use the HTTP Accept-Lang header? That is somewhat of a solution for those that know to configure their browsers (very few people). Otherwise I think English is the best for now.