The real authority on this is ICAO Annex 10, Vol II which says:
5.2.1.2 Language to be used
5.2.1.2.1 Recommendation.— In general, the air-ground radiotelephony communications should be conducted in the language normally used by the station on the ground.
Note.— The language normally used by the station on the ground may not necessarily be the language of the State in which it is located.
5.2.1.2.2 Recommendation.— Pending the development and adoption of a more suitable form of speech for universal use in aeronautical radiotelephony communications, the English language should be used as such and should be available, on request from any aircraft station unable to comply with 5.2.1.2.1, at all stations on the ground serving designated airports and routes used by international air services.
My reading of that is that you should use the language of the folks on the ground. Failing that, try English! 