Collaboration within multinational teams necessitates the adoption of a common language, typically English, which often leads to significant differences in language proficiency across members. We develop and test a multilevel model of the effects of language proficiency within multinational teams. An experimental study of 51 teams (102 American and 102 Chinese participants) revealed that, at the individual level, members with higher levels of language proficiency were more likely to speak up, which led to more positive perceptions of their competence. At the team level, greater dispersion in language proficiency across members was associated with less accurate competence recognition, which, in turn, led to lower overall team performance. Moreover, communication medium moderated these relationships, such that the effects of language proficiency were more potent in face-to-face than in computer-mediated teams. We discuss the implications of these findings for future research and for managing participation, competence, and technology in multinational teams.