You have no idea what you're talking about.....especially with americans flying out of north america,for example europe....they have trouble to understand english without "the american" accent, for example british pilots and australians have absolutely no problem to undestand correctly spoken english,but without american accent....so I wouldn't say that there is problem with pilots poor english knowledge.....as an ATC I know what I'm talking about,because from my position,as a one who is instructing pilots it is just simple....because pilot is responding,I'm familiar with my airspace,I know all waypoints and procedures,which is not case for pilot,who flies through my airspace maybe 10x per year,so he cannot remember it,and thus react promptly on my instructions.....and this happens with americans over my airspace also....especially when I use ICAO phraseology,they have problem to understand....just because they are used to slang and "open language" or some kind of freestyle....over US airspace...I just hope it won't result in accident somewhere over congested area in US...
Might one hope that ATCO's English is more "correctly spoken" than the above is written? (If not, difficulties in understanding could be entirely understandable.)
It is also worth noting the the British and Aussies (as well as all English speakers) have accents, as well. Is an American's difficulty in understanding a British accent any different than the converse?
And lest anyone try to lay claim to the "pure" version of the language, it should be remembered that all language evolves. Perhaps the purest modern variant of the "King's English"
might be that which was left with the colonials across the pond, a couple of centuries ago.