Context: In order to gain competitive advantages in the global market, many software outsourcing organizations in non- English speaking countries adopted the use of English as the lingua franca mandate in their organizations. Goal: We sought to assess the influence of the English lingua franca mandate on the teamwork in software outsourcing vendors empirically. Approach: We conducted a field experiment performed in a Chinese software outsourcing company. We measured the team-work related constructs repeatedly with the progress of field experiment process. Results: This study reveals enforcing the English lingua franca may lead to: (1) lower satisfaction at work, (2) the decrease of teamwork quality, and (3) the lack of coordination in terms of social-technical congruence. Conclusion: Decision makers may need to be careful before launching the English lingua franca mandate. We also conceptually discusses how the English lingua franca mandate's influence on software quality. In a broader sense, the research calls attentions to the ''language'' issues in global software engineering practices, which is well-known but has not yet received enough attentions in software engineering research.