Funny how an airprox in Bacelona diverts to pros and cons of Using French in Canada ... But one point you have right :
it's about culture and politics.
Gentlemen, ICAO rules of the air specify which language to be used when overflying a State. ( the language set by the State ) All attempts to change this since 1944 have failed all miserably.
No-one wants to try again. Check with your State ICAO commissioner if you do not believe me .
English Language proficiency testing is of course one step forward, but does not solve the use of 2 languages simultaneously..
Using English everywhere is of course and by far the best option. But not necessarily the safest in places where nobody else speaks English, ( e.g Russia to take only one example) and contrary to popular belief it is not a high safety risk. (and before you start shooting , check your facts and figures )
Over and out.