Informal communication in organizations has many benefits, but people who are not native speakers of the organization's common language may find it hard to interact informally. In an interview study of nine native English-speaking and 33 non-native English-speaking students at a large U.S. university, we explore how native language shapes patterns of informal interaction. We found that non-native speakers generally preferred interacting informally with fellow speakers of their own native language as opposed to native English speakers, which hinders communication and collaboration between groups. Three factors led to this "clustering" effect: issues of common ground, feelings of social obligation to other speakers of one's native language, and desires to build social networks within a language group. Four factors led to greater motivation for cross-language interaction: a desire to build bridging capital, physical proximity, one-on-one or small-group interaction, and an established work relationship. The findings suggest ways that communication tools might reduce barriers to informal interaction between speakers of different native languages.