Is English sufficient for international aviation?
ATC and pilots must be able to communicate in English on international flights involving ICAO member states. There are 191 countries that are members of the ICAO, covering almost all of the world.

While communications may be performed in the local language, English must be used upon request on international flights. Controllers and flight crew engaged in communications for international flights should be proficient in spoken English in addition to English aviation phraseology. 

Some domestic airways and airports may require communications in the local language instead of English.

So English is sufficient if you stick to international airports and airways, but be sure to check the language requirements before you venture into foreign domestic airspace and airports.