In EFL contexts where students' chances to use English are limited to the classroom, minimizing their use of L1 to develop fluency in English is often encouraged in instruction. However, recent studies have reported that students' partial use of L1 in code-switching or translanguaging offers various pedagogical advantages. This research examines advanced Japanese high school students' use of and perceptions toward L1 (Japanese) in translanguaging during communicative L2 (English) activities. Drawing on both quantitative data from a survey answered by 190 third-year high school students and qualitative data from classroom observations and in-depth interviews with nine students from the same group, the study attempted to reveal context-sensitive findings about students' use of L1 in translanguaging in the EFL classroom. The quantitative survey results revealed that all the students, to varying degrees, partially used L1 during communicative L2 activities. Data from classroom observations supported the survey results and identified five salient speech functions of students' partial L1 use in the activities. In addition, the interview data showed students' nuanced reasons for and perceptions toward their partial L1 use during such activities, highlighting a unique communication layer for translanguaging. Based on the discussion of the findings, recommendations for pursuing contextualized communicative language teaching and translanguaging pedagogy in EFL classrooms are provided at the end of the paper.