A heavily tattooed man stripped off his shirt and donned a pit-bull mask in front of the sweaty crowd. He was about to rap in Afrikaans—to fans in New York.

"Hi, my name's Ninja, and I'm from South Africa," the frontman for Die Antwoord said later, before launching into "Baby's on Fire," a song that makes liberal use of Afrikaans slang like "cherrie" (girl) "klapping" (slapping) and "lekker" (awesome).

...