As far as I can see it can only be with reference to the use of the radio. I have to say we need to keep the medium to one language. It is a common channel and you have to be understood clearly. I often hear it here in the WC that guys are having long chats in Afrikaans on 124.8 about where to land and where to meet, what conditions are like and where they are going. This is all very well but if a foreigner is flying in the area he will not understand any of this valuable, and sometimes non-valuable information. (I am not knocking afrikaans per se, it is just an example, please don't attack me on this one:shock: )
Secondly you must not only understand the language but you must also be clear in speaking it. I know a lot of people who can understand a language but not necessarily speak it . However your language ability should be sufficient to cover radio specifics, i.e. understanding the accepted word usage, terminology, etc and not required to be able to address a symposium of the English language.
But then all of this should be covered in the Radio Licence. It is therefore imperative that non-english speaking foreigners pass a Radio Licence exam with the emphasis on language proficiency both from an understanding what the ATC is saying to you as well as being able to make yourself absolutely clear to others.
Stepping off box
