188.8.131.52.1 The air-ground radiotelephony communications shall be conducted in the language normally used by the station on the ground or in the English language.
Note 2. The level of language proficiency required for aeronautical radiotelephony communications is specified in the Appendix to Annex 1 (ICAO Language Proficiency Rating Scale).
184.108.40.206.2 The English language shall be available, on request from any aircraft station, at all stations on the ground serving designated airports and routes used by international air services.
220.127.116.11.3 The languages available at a given station on the ground shall form part of the Aeronautical Information Publications and other published aeronautical information concerning such facilities.
18.104.22.168 Word spelling in radiotelephony. When proper names, service abbreviations and words of which the spelling is doubtful are spelled out in radiotelephony the alphabet in Figure 5-1 shall be used.
Note 2. The Spelling Alphabet specified in 22.214.171.124 is also prescribed for use in the Maritime Mobile Service (ITU Radio Regulations, Appendix S14).
Source: ICAO Annex 10.2