When interacting with other cultures we expect there to be communication barriers due to differing language, but people are less likely to anticipate nonverbal differences, often times resorting to use of hand signals or gestures that may infact represent somethign totally different in other cultures.
We talk a lot in class about how the middle finger in America is a negative thing, whereas in other cultures the middle finger is used as simply a pointer finger or something else that is less confrontational.
I havent had much personal experience with this but simply as a barrier I think nonverbal communication is the most tricky thing, because language's meaning can be found in definitions, but nonverbal communication has much to do about social cues and simply being in a culture. They are much harder to pick up on and understand as they as so integrated in a given culture and aren't taught in conventional ways, like a language can be taught.