On March, 5 2011, pilots and air traffic controllers working at international airports will be required to pass the International Civil Aviation Organization (ICAO) English exam with a score of 4 or better (find a definition of the ICAO scores here). LOI English is now offering aviation English classes to help pilots and air traffic controllers prepare for the test.
To pass with a 4, a fairly high level of English fluency is required. As of now, pilots and air traffic controllers must be able to speak English upon request, but as of March, English will be the universal language at all international airports. Over the years, there have apparently been a number of communication errors that have caused accidents or near-accidents at airports. Having a universal language requirement at all international airports should help prevent airport communication problems.
The YouTube video below captures a misunderstanding between a Swiss pilot and a Russian air traffic controller, and illustrates the need for a common language at international airports.