Translanguaging offers a new perspective on language learning by affirming and leveraging the diverse language practices that make up learners' unitary language repertoire as resources for their learning. Despite the potential pedagogical benefits of translanguaging, English-only policies are still prevalent in many language classrooms. Even when translanguaging is welcomed into the classroom, the conflicting attitudes of teachers, students and families pose ideological constraints on translanguaging which restrict learners from selecting and utilising features from their whole translanguaging repertoire. Guided by translanguaging and sociocultural theory, this study examines the tension between the affordances of student-led translanguaging in a Grade 5 Malaysian classroom with an English-only policy, and the constraints to learners' use of translanguaging. This paper reports on the results of a sociocultural critical discourse analysis of learners' peer interactions while engaged in collaborative learning, and interviews with 31 learners. The findings indicate that learners used translanguaging agentively to support one another's language learning, build rapport, resolve conflict, assert their cultural identity, and draw on knowledge across languages. However, learners' use of translanguaging was constrained to an extent by their teacher's and peers' language policies and practices, parental discourses about linguistic capital, and societal discourses on ethnicity, nationality, and marginalisation.