English is probably one of the easiest language to learn, I speak 4 languages, and I can tell you French and Japanese are more complicated then English.

There's a reason why it's one of the most spoken language worldwide, and it's not because it's hard.

They probably stopped talking in French:
a) to have a better group communication
b) to avoid unnecessary flame war, since some of English only people get frustrated to see someone speak in other language because they don't understand it.