User blog comment:Chinese-American Empire/Why does Wakanda speak english?/@comment-26218526-20180726165747

Like Angel Gro said, I don't think there's an in-world explanation. Aside from that, they probably use English so you don't have to read subtitles for the entire movie. And if they only spoke Wakandan, they wouldn't be able to interact with Ross, Klaue, Killmonger, the Avengers, etc.