But also making fun of people behind their back is probably not good either. Personally whenever I can’t understand someone at work I just straight up tell them I can’t understand them. I don’t explain why (unless they ask), or grant sympathy. The basic expectation of the workplace is the ability to communicate clearly.
However, if the populace is inclined to make fun of you, you probably deserve it one way or another. Typically people mock features or habits of people that are possible to change. For example, anyone smart enough to be at Facebook is smart enough to fix their accent. Your coworker simply sounds unsocialized and poorly acclimated to western culture. To me, he should be mocked, because he’s perfectly capable of changing and should do so. Of course, behind the back mocking doesn’t do anyone any good. Your friends have no spine for not pointing out to this man he has bad habits that cause him to lead a sub-optimal social life.