I just looked it up, and it looks like the literal meaning of "nani" is "who": https://en.wiktionary.org/wiki/nani#Swahili "What" is "nini": https://en.wiktionary.org/wiki/nini#Swahili

I did see "who" listed as a translation when I moused over, but I asked the question anyway because things are often translated contextually on Duolingo.

It's just interesting because I've also seen the question asked this way in Indonesian/Malay: "Siapa nama anda?" ("siapa" also meaning "who").