I wouldn't say that the language is a major problem. When you start studying for this test you know it's gonna be in English, so I don't think that people would consider taking it if they didn't have a firm grasp of English. Anyway, the terms involved are mainly medical and rather simple. I mean, one doesn't have to have a profficiency in English to understand what's being asked.
I wouldn't also think that the curriculum is a big problem. Maybe THE WAY we are taught things is different. I for example never looked at the"clinical feautures" of any disease while studying pathology for my course exams, because noone was going to ask about them, although I did go through Robbins cover to cover. On the contrary, most of the questions in step 1 were exactly about these paragraph-long clinical features. Not only in pathology, but in general.
The other thing is the MCQ format of the test. I firmly believe that plainly memorizing is much harder than having to face MCQ'S. One could argue that MCQ'S can be made very hard, and I would agree on that. However, the purpose of USMLE isn't(in general) to trick you,and statistically you will have some questions right anyway, so this argument doesn't have a big value here.So why IMG's do worse than AMG'S in a theoretically easier form of examination? BECAUSE THEY ARE NOT USED TO IT. Laugh if you have to, but I know a lot of people that claim that they had to re-learn basic sciences backwards, and I'm one of them. It's simply very difficult to deal with an exam form you probably have seen only a couple of times(if ever) in your life, and even more difficult to compete against students who have ;earned everything they know following this form. Personally I think that the American approach is better(and much more fun) than the European, but this doesn't mean it's harder, it's just...different.
Finally, I think that there's a psychological issue too. American students HAVE to pass this exam while IMG'S are looking for an option that will bring them to the next level. I mean, sure, US is undoubtedly the best place around to get your training done, but it's not a dead end for an IMG, it's just the best option available. For an IMG to pass this exam is a big success, while for an American student is an OBLIGATION, which says a lot about the motivation of each group.