Pilots fly all over the world, and to do their job correctly, they must communicate with local air traffic controllers wherever they go. The only way for the system to work properly is if everybody involved can speak the same language.
International treaties have designated English as the official language for airplane communication, though airports may use another language if both the pilot and the controller can speak it. The controllers must speak in English if the flight crew is not comfortable speaking the native language.
Unfortunately, many pilots only learn enough English to get by, leading to occasional misunderstandings. It's not clear how many accidents and incidents are the result of language barriers, but some studies show that the problem is getting worse.