Well the worst ever air disater, Tenerif, happened due to miscommunication, loss of situational awareness, all this happened using the ENGLISH language. So it does not really speak for the one language only theory.
And why always the French?
Have you been to Spain, Mexico, South America, China, Canada (Montreal sector), parts of Africa, Russia, please ask them to speak only ENGLISH. Good luck.
And then I rather have two people using their common native language and understanding each other than to do it in a language that is not theirs and failing to communicate effectively...