We report on U.S. middle school Arabic‐speaking students who were agentive in translanguaging for disciplinary learning during social studies inquiry and suggest specific ways that students' academic learning and bilingual development can be supported through translanguaging in the context of English‐dominant classrooms. We engaged in design‐based research with teachers and students over five investigations and refined our mediating processes as we encountered challenges in supporting translanguaging in this context. Analysis of video records showed that for learners to be agentive in talking about subject matter concepts, they needed register development—not only in English but also through translanguaging. Bilingual inquiry curriculum materials and encouragement are not enough; interaction with others who can discuss disciplinary concepts through translanguaging is needed to support learners' agency in drawing on all their meaning‐making resources. The presence of recently arrived learners new to English creates contexts for translanguaging, but even fluent bilingual learners benefit from further development of Arabic social studies registers. With ongoing practice, emergent and fluent bilinguals participated in inquiry practices through translanguaging. Support for translanguaging in the context of learning school subjects can enable learners' development of their bilingual meaning‐making potential.