Dual language communication is not a safety issue as ATC never ever get anything wrong and Pilots never need to know what any other aircraft is doing ....ever
I have submitted 3 MORs at french airfields due to loss of situational awareness caused by ATC issuing critical information in French only. (eg, closure of our 2 diversion airfields, Orly and Beauvais, due to thunderstorm activity!!).
If French was the language of international aviation, that is the language I would use as I believe that flight safety overrides any jingoistic tendencies.