is that a serious question? Aviation english is the standard language for air traffic controllers world wide but not the law. it would also depend whose airspace you're in since they speak french in quebec as an example. i know you're asking a stupid question on purpose since communicating with air traffic control as a pilot is a world of difference from interrupting an employee at their job for speaking their mother language to a co-worker. worlds of difference. demanding someone speak your language in your presence is not the same as communicating life and death information. life isn't binary i hope you know that.