Students’ identities are connected to their productive participation in the mathematics classroom. A bilingual Turkish-German teaching intervention intended to foster the students’ conceptual understanding of fractions has to account for the students’ identities, since the students’ identities within the intervention influence how the students utilize the learning opportunities. To account for the dynamic and interactive nature of identities, positioning theory was applied to reconstruct the students’ identities as multilingual mathematics learners in four teaching intervention groups with four different teachers. In each group, students developed different identities as multilingual mathematics learners, ranging from “student in need of help” to “student responsible for mathematics.” These identities were differently affected by the Turkish language. The analysis indicates that Turkish becomes a resource in the mathematical conversations when the students collaborate towards a consensual solution and are made responsible for each other’s understanding. As a consequence, for developing teaching interventions aiming at building on students’ multilingual resources for participating in mathematical discourses, the ways in which students can develop identities must be taken into account in order to enhance productive engagement.