Upvote:6
The answer is in the question. Ryanair believes that many people who present South African passports are not entitled to them.
Due to the high prevalence of fraudulent South African passports,
Unfortunately, someone in charge of trying to figure out whether passports are fraudulent or not took a weirdly racist shortcut of "if you're a South African citizen, you will understand a language the majority of South Africans despise and hate, and be happy to answer trivia questions in it".
So the answer to your question of "why?" is
More details in this CNN article, including the confirmation that the UK government doesn't require anything like this.