In 1976 BEA lost a Trident in a collision over Zagreb which could have probably avoided if English had been used.
We haven't come far since then especially in the south of France where I have painfully listened to a couple of approach controllers who had difficulty in understanding Korean, French, American, Scandinavian and Irish pilots communications in English.
I worked for a Swiss German company - Switzerland has four official languages and we did everything in english except cabin crew briefings.
It worked well except for some of the militia pilots whose english was on a par with the French - inadequate but as their mates tested them they got through the system.
One language - anything else is pure stupidity.