If you want to fly an aeroplane, you have to speak English.
This isn’t a joke, as this rule was established after a terrible accident that happened at the Tenerife airport in 1977. Two airplanes collided because one of the pilot’s English wasn’t clear enough for the tower to understand what he said. Today if you want to be a part of aviation staff you need to take a proficiency test that includes aviation terminology as well as the capability to understand English when there are obstructions or interferences, as often happens over radio.