As I see it there are three possibilities.
1. Everyone speaks the same language, the world over, in order to have any level of pilots license, and all communication on all frequencies in all airspaces is in that one language.
2. When in X airspace, everyone speaks Xish.
3. There is more than one language spoken on the frequency.
I don't think #1 or #2 is feasible.
Agreed.
In controlled airspace, we have to accept both English and Xish. Those pilots who don't understand Xish will have to either learn Xish, or rely on the controller for situational awareness. It is not ideal, but it is the only practical solution.
In uncontrolled airspace in country X, only Xish should be used. If you can't speak Xish to ICAO level 4, then you should not be in uncontrolled airspace.