It’s well known that isiZulu is the most widely spoken language in South Africa, with close to 12 million people listing isiZulu as their mother tongue, but that number jumps dramatically when you include second-language speakers, with a staggering further 15 million people saying that they speak isiZulu as a second-language.
One possible reason for such high numbers could be that isiZulu is very similar in structure to other languages that are categorised by linguists as Bantu languages, in particular isiXhosa, which has just over eight million first-language speakers. isiZulu is also closely related to isiSwati (Swaziland), and isiNdebele (Zimbabwe), making it the lingua franca from KZN to Zimbabwe.