As I understand it, strictly speaking, "Aviation English" is only a recommendation from ICAO (International Civil Aviation Organization - the UN Agency that coodinates this stuff), not a requirement. But, in practice, it's a de facto rule that commercial airline pilots and air traffic controllers be proficient in it.

It is, however, common practice for domestic flights to conduct communications with ATC in the local language - they might have to be *able* to use Aviation English but (unless local regulations/law etc. says otherwise) they don't necessarily have to do so if the flight crew and controller have another language in common.

That one is one of those things that I actually know, but don't know how I know.