Although I've been retired for more than a year now it was my impression that "situational awareness" was decreasing anyhow - at least in the upper airspace, so it is to some extent immaterial in which language ATC instructions are given. The trend toward larger sectors,with horizontal splits, higher traffic flows, direct routings, position reports a thing of the past, RVSM, and the gradual introduction of Data Links have all contributed. This has been partly offset by the introduction of TCAS, but this is still not a total solution.
I can remember in the 1970's the Canadian controllers'action, Ėnglish is the language of International aviation"when Quebec tried to enforce bi-linguallity (i.e. everybody must speak French) throughout Canada (this was to have extended to all flight manuals being bi-lingual, imagine carrying two brain bags). In Belgium the same practice existed, although there are 3 official languages there, French and English being allowed on the R/T until about 1974. For a while I had to do this in schoolboy French, and a number of Walloons insisted on this right until the very end.
There were (are?) discrete frequencies in come countries, e.g. Germany, where the native language could be used for VFR flights. Not the best idea when one controller was working more than one freq. and I remember being told in English, to line-up at Caen when a French speaker was on short final (OK not neccessarilly 100% due to the use of two languages) but as Redsnail pointed out learning a few words, climb, descend, turn, in a number of languages may save your bacon one day. And if an Oz-mate can do it anyone can
. Also when wx allows there is still no substitute for the Mk. 1 eyeball.
(Please excuse the ramblings of an OAP, however not quite senile yet)
Lon More, here before Pontius was a Pilot or Mortus a Rigger