Main Dog,
You make good points and you're obviously one of the more aware pilots on the air.
My main point, however, is that in a terminal environment, more often than not, the departure traffic will be segregated to another frequency than that of the arrivals. There are terminals with 4, 5 or even more frequencies being used.
What you described is obviously hampered by the use of two languages but in the end, altitude busts usually happen between departures and arrivals and seldom between airplanes that are following each other on TCAS regardless of language.
So while you might be fretting (or not
) about the traffic in front being in french, there might be an english speaker coming right at you on another frequency and you'll never know.
I can see one language being mandatory at an uncontrolled airport where comms and SA are the only separation you'll have but, where a controller is involved, I do believe that the best situation is when the controller can understand and be understood by the traffic he or she is controlling. If I'm in China and a Chinese pilot is making a run for me, I don't want the controller to try to speak english to him; I want him to get the message across quickly and clearly.