English is the "official" language of aviation. As far as I know, there is no place where you can't legally fly if you don't speak the native language. Local pilots flying in local airspace can and do speak the local language, but when they enter a new area where another language is spoken they must speak english. Personally, I fly into Quebec City and Montreal at least once a month and it is common to hear ATC in both english and french. From an operational point of view, the problem with having english as the official language is that one contry's definition of fluency may not be the same as anothers. I've seldom had a problem understanding western european controllers, but eastern european, asian, and Boston Center
controllers can sometimes be quite challenging. I am (or used to be
) fluent in spanish. When operating in latin and south america I have been known to speak spanish to the controllers when their english was just too dificult to understand. I hated doing it because it effectively put the other pilot "out of the loop", but better 2 out of 3 than no one. This past year we spent a week operating into a few "domestic" Brazilian airports. We found it easier to find an airline employee who spoke excellent english and pay him to fly with us for a few days. It sure made things easier.