Note: Please upvote the question too!
Good question. No explanation as of the Season 2 finale. But, this is explained in the Season 3 premiere: (Maybe they made this explanation BECAUSE of this question. LOL. Edit: See below!)
His native language is French, but he went to American schools. He says
I grew up in Senegal so my native language is French. But I went to American schools, so I also speak English.
Here are some screenshots:
This may have been different in the reality where they all died as they originally did, but you're asking about the reality where Chidi speaks English.
Edit: Actually, that the season 3 premiere addresses this question is mentioned in some articles: 1, 2 (this article claims this Chidi thing to be a 'massive' plot hole. lol 'massive' really?), 3. Apparently, a lot of people had this question at the end of s2.
Actually there's another question that one of the above articles asks
Why does Chidi have an American accent, then?
The same article gives an official answer:
Well, according to The Good Place's creator Michael Schur, it was just a helluva lot easier that way.
- My thoughts: Or he just has a good American accent. There are lots of multilingual people who are very good in multiple accents, like they really sound American when speaking (American) English and then they really sound German when speaking German. See this Code Geass case for example.