Sorry - my post wasn't clear. I agree that a pilot born and raised in X, who only speaks Xish, should be allowed to fly in all airspace (controlled and uncontrolled) in country X.
However if he wishes to fly in controlled airspace in country Y, he should have at least ICAO level 4 in English (or Yish). If he wishes to fly in uncontrolled airspace in country Y, he should have at least level 4 in Yish.
So, he's flying in his own country, X, speaking Xish, and other people are speaking English on the same frequency at the same airport, and we are back to the same problem. Two languages being used at the same time.
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.