Well, I live (and fly) in an interesting part of the world, linguistically speaking: Canada. Quebec, to be specific. Though just a PPL, I have flown a significant amount of time in controlled airspace.
You have to factor something into the language equation. Not everyone can master English in many parts of the world. In particular, at the "low end" of aviation, light GA or bush flying, the knowledge of English in Quebec is fairly limited. Although to work in ATC you have to be bilingual, many controllers speak with a pronounced French accent. Throw in a PPL holder who can barely understand English, and I fail to see how comprehension or situational awareness, for that pilot, will be enhanced by forcing English. It's a reality we have to live with.
As I'm fluent in both English and French, it's less of an issue for me. But for others, it's a problem.
In any case I don't know of a single accident in Canada that can be attributed to the language issue. Our controlled airspace is perhaps not as busy as in the US or Europe, so ATC has more time to sort things out with improving situational awareness for two unilingual pilots of opposing language.
I've heard the flow go so rapidly from "Tango Lima Mike, turn left 2-4-0, your traffic is an A320 at 3 o'clock, three thousand; Air Canada 173, vous avez un Beechcraft à 9 heures, 2000 pieds" so smoothly it doesn't matter (yes, Air Canada pilots can and do use French). No doubt if all English, ATC would give exactly the same amount of wording, except all in English. And then one of the two aircraft involved might not clearly, or as quickly, understand the ATC instructions.
There were huge protests and predictions of carnage in the skies when French was allowed into ATC coms, first VFR only, now VFR and IFR, in the early '80s (when I learned to fly). It has not come to pass. The old rules of "see and be seen" still apply for VFR, as does positive traffic separation for IFR. I think it's actually more of an issue at uncontrolled fields with Mandatory Frequencies, than in controlled airspace. But then poor communications practices happen at uncontrolled fields regardless of the language being used.
Mike