This study examined how middle-grades language arts teachers learned to integrate a small-group collaborative translation activity into their teaching practice. We discuss what we call "pedagogical translation" as an emergent social practice, in which translation routines that are familiar to multilingual students may be leveraged toward instructional goals in a mainstream language arts class. The data were drawn from a classroom teaching experiment iteration of a larger design-based research study, whose goal is to create a fully developed instructional protocol useful to all teachers in linguistically diverse language arts classrooms, but especially teachers with limited or emerging proficiencies in languages other than English. We position pedagogical translation as a paradigm case of translingual pedagogy--instructional approaches designed to leverage the full range of emerging bilinguals' linguistic resources--and we focus our analysis on the agentive participation of teachers as they integrate new translingual routines into their instructional practice. Using a conjecture mapping procedure, we describe the evolution of an instructional theory for how pedagogical translation can be leveraged toward literacy learning objectives. We present qualitative narratives describing how participating teachers made locally situated design choices that meshed new routines with existing instructional practice, documenting trajectories of teacher participation as agentive designers of translingual pedagogy.