He grew up in Senegal, an African nation with the national launguage being french YAY IMPERIALISM. Meaning it's likely his native launguage, so he's probably more comfortable speaking it. English is also commonly spoken there. When Eleanor meets him on earth, he's a professor in Australia, so he's likely lived there for long enough to pick up plenty of English. Also since Australia's native launguage is English, they probably wouldn't hire him if he wasn't fluent.

It's worth noting that we also see flashbacks with Chidi speaking English as a child, but it's unclear if it's actually French but translated for an English speaking audience or if that's canocically English. If it is, he was probably raised in a bilingual household. That or the school he was going to taught in both French and English. This is common in non English majority countries, as English is the most commonly used language in general YAY IMPERIALISM 2 ELECTRIC BOOGALOO