In the MCU, the inhabitants of the nation of Wakanda largely speak English (although other languages, such as Xhosa, Zulu, and Igbo, are also heard). It's not simply that they are cosmopolitan and happen to know English: this is the case even when people are speaking among themselves, not foreigners. That said, in real life, countries where English is the dominant everyday language tend to be either the UK or former colonies of the UK, for obvious reasons. Wakanda was highly insular, never colonized, and quite independently wealthy, which would seem to disfavor most reasons to adopt English as an everyday language.
Obviously, out-of-universe, most of the actors were American, and most American audiences would not understand the other Wakandan languages (to say nothing of a conlang). However, in-universe, why is English the predominant language in Wakanda?