ryu2 : ICAO annex 10 says that the language to be used in R/T communications is the language of the country overflown. When no agreement exists between pilot and controller, R/T based in the English language shall be used.( or made available )
This is the reason why most countries have their national language as prime R/T language, English second :
Largest examples : Whole south and Central america ( Spanish ) with the exeption of Brazil ( Portugese ) Canada ( French in Quebec Province ) Russia and most of the CIS ( Russian ) , mainland China ( Mandarin ) , most of the North and west Africa ( French ) and most south European Countries ( France, Italy, Spain, Greece, etc..) use their own national language first.
The North European countries , including Germany have abolished the use of their National language in favour of English only , but only in IFR and at their International airports ( they mostly still use their national language for VFR anfd at local airfields. )
Hope this helps.