Recently there has been considerable interest in the role of first language use in second/foreign language learning, especially where students share a common first language. However, most research has focused on in-class interaction between teachers and learners. Much less attention has been given to students' out-of-class practices, for example, in collaborative project-based learning. To fill this gap, the article tracks the out-of-class activities of 16 students (four project groups) involved in project work on a course in English for science students at an English-medium university in Hong Kong. An analysis of students' computer-mediated interactions (Facebook, WhatsApp and email) shows that these interactions are plurilingual, with students drawing on English, Chinese and mixed code to different extents as they go about their project work. Different languages are used strategically: whereas L2 is used more in the construction of the final project product, L1 is used more to promote group cohesion. The findings suggest that, in plurilingual contexts like Hong Kong, it is necessary to develop an English language pedagogy that acknowledges the need for the constructive but judicious use of translanguaging and plurilingual practices as students are engaged in L2-focused (e.g. EAP) project-based group work.