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...