All learners bring ideas about science phenomena to classroom learning, including formative assessment tasks. Educators and scholars have long been interested in making school science, including assessment, more equitable and culturally meaningful for (bi)multilingual learners. Translanguaging is increasingly seen as an important assessment design principle in (bi)multilingual classrooms, including science classrooms. Despite the increasing popularity of translanguaging as a pedagogical and assessment tool, questions remain about how teachers co-design and interpret translanguaging on formative assessments when teachers do not share multiple linguistic resources with their students. This manuscript explores the journey of one experienced and highly qualified teacher, Emily, as she participates in a teacher-researcher co-design collaborative focused on inviting students to draw on and deploy translanguaging in science formative assessment. I qualitatively analyze illustrative conversations during the "Reflect and Modify" phase of four sequential (Trans)Formative Assessment Co-design (TAC) cycles. Findings point to the ways in which Emily began to develop translanguaging interpretive power to understand (bi)multilingual work as well as the confidence to lean into the knowledge she brought to the conversation. Ultimately, participation in collective conversations about (bi)multilinguals' work supported Emily to expand her ideological stances into more concrete, agentive actions.