This is only anecdote, but I had a student from China this year whose spoken English seemed very poor (partly through nerves), but who was able to understand medieval English to the extent that she could explain to me in detail how certain tropes were also present in Chinese folk tales - which, frankly, blew me away.
I accept that students studying in foreign countries need to reach a minimum standard, and if they don't, they can really make life tough for their coursemates. But, I think students can be quite unforgiving and insular, too.
When I was doing my teaching practice, we were put into groups and asked to prepare a lesson targeted at the other students in the room, whose subjects ranged across arts, sciences and humanities and who came from several countries. My group was the history group, and we were to prepare a lesson on the Black Death. One girl insisted that everyone would know what that term meant, and thought we should not bother to say what historical period we were talking about. I pointed out that there were students from the US, from the Middle East, from India and from China, who would have good reason not to know, and she smirked and said that even in China and India, 'of course', they study the Black Death in England. 
Of course, they didn't, and of course, she looked extremely silly when she tried out her theory. I have also come across people who are shocked that non-UK students are unaware of the significance of 1066. This is extreme, but often, students who think other non-UK students are shockingly ignorant, are actually showing up their own areas of ignorance.