Isn't English the language of all pilots?
Nope. Only those working within civil aviation - proficiency levels determined by the International Civil Aviation Organization (ICAO).
In many countries, military pilots have no English language requirements, although most non English speaking countries encourage their pilots to reach ICAO Level 4 which is usually enough to communicate with foreign ATC.
The problems start to arise when you get two non native English speakers communicating in English - if one has a poor level of English it is very easy for misunderstandings to arise, and that's putting it mildly!
JF