Have many non-native English speaking friends, I understand their difficulty with the language. When communicating with my friends I try and choose appropriate words to avoid misunderstanding until I can grasp the level at which they understand English. It bothers me when I see other people communicate to non-english speakers and they either get irritated or they don't understand how what they said could be misunderstood. It is as if they've never evaluated the difficulty of their own language.
Then it gets even worse because in casual conversation people will throw in all kinds of slang and just totally made up words. As native English speakers you can pick them apart and get an idea of what's happening even if you don't know the specific slang. A non-native speaker has no hope at all.
With all communication it's important to know your audience and address them in a way they can understand.