User blog comment:Chinese-American Empire/Why does Wakanda speak english?/@comment-4478016-20180726163544

There is no in-world explanation, except for those of the Elite/ Royal family being public figures, thus requiring English, as it is used commonly as an international language, for things like public relations, trade and commerce, and treaties. This too could make sense for the Border Tribe, of which W'Kabi is part of, as many tourist may be lost, hunters hunting (obviously), etc.

As for the real-life, the explanation is that the average american citizens doesn't know languages outside of English, and they dislike reading subtitles as they distract from paying attention to what's happening.

Hope this is a good enough explanation for you! :)