Chuks,
English may not be the official language of aviation, but the International Civil Aviation Organisation has decreed that from 1 January 2008 all Air Traffic Controllers and Flight Crew Members engaged in or in contact with international flights must be proficient in the English language as a general spoken medium and not simply have a proficiency in standard ICAO Radio Telephony Phraseology. The regulations are contained in Annex 1, Chapter 1, paragraph 1.2.9 and Attachment to Annex 1, and also to Annex 10, Volume II, Chapter 5.