The most likely answer for this blank (keeping in mind that your text or instructor’s lectures may have a specific answer you are expected to give) is the word “culture” or “cultural.” Misunderstandings can arise between project members because they do not share a common native language and because they do not share common cultural expectations.
If the team members are physically in different countries, the cultural problems are most likely to come from the ways in which language is used. The various team members may be using words whose literal meaning is clear to all but whose cultural nuances are not. For example, in Japanese culture, there is still something of a taboo on being too clear about one’s meaning when asking for something or when refusing a request. If a Japanese person, then, were interacting with an American, the American might come away with the idea that the Japanese person had agreed to a proposal when the Japanese person thought they were politely evading the question in a way that expressed their disagreement with the proposal.
Even when people are relatively adept at speaking a given language, cultural differences can make it hard for them to actually understand what the other person means.