Due to the fact that lack of language proficiency was a contributing factor to a lot of accidents the International Civil Aviation Organistaion (ICAO) has developed and implemented the "ICAO Language Proficiency Requirements".
As of 5 March 2008 pilots and controllers involved in international operations therefore are required to demonstrate their proficiency in English at least at Operational Level 4. In ICAO Document 9835 (Manual on the Implementation of ICAO Language Proficiency Requirements) a rating scale and a minimum skill level for language proficiency have been established.
ICAO has defined six levels of language proficiency beginnig with Level 1 (pre-elemantary) up to Level 6 (native speaker). Operational Level 4 is considered the minimum level of proficiency for pilots and controllers.
In order to demonstrate their speaking and listening abilities according to at least Level 4 pilots or controllers have to undergo a Language Proficiency Test which is conducted either by the Bundesnetzagentur or a Language Testing Organization (LTO) accredited by the Luftfahrtbundesamt (e.g. the DAeC) or single accredited examiners.
According to German Law (LuftPersV § 125) only pilots of aeroplanes, helicopters, airships and blimps are concerned as far as they conduct radiotelephony in english in german airspace or abroad.
Pilots holding a licence for gliders and microlights are not concerned.
For pilots flying VFR the demonstrated Language Proficiency has a validity of four years, for pilots holding an instrument rating three years. Thereafter a recurrent test is necessary.