One Language
For safety reasons, and nothing else, there should be only one language in aviation. Presently that language is English. End of story, or is it? One could argue that a PPL on a local VFR flight flight should be allowed to communicate in the local language, as indeed is the case in many if not most countries.
However, a vast number of native English speakers have sadly fallen into the trap called "complacency" with regards to learning a foreign language. They feel they don't need it, since "everyone speaks English". The sad part is that they probably don't know what they are missing. I am writing this in what is my 3rd language. Danish being the first, German the second. Those 3 languages are mandatory from a very early age in school, and you will have to take English and either German, French or Spanish up to and including High School. I speak 3 languages fluently, and you could not begin to imagine the benefits it brings when travelling. Further, I am almost fluent in Swedish and Norwegian, understand a fair bit of French and Spanish, and am picking up on Dutch/Flemmish and Italian.
Now if the English were forced to take up language training in school, maybe they would not be quite so challenged when aviating in foreign skies, and would perhaps stop the endless and quite honestly disappointing bashing of anything "non-English", especially the French. The French have a deep understanding of the importance a language has on culture, and ferociously defend French. As well they should.