We worked to identify how the availability of two languages, Spanish and English, facilitates the teaching of mathematics when instruction is centered on students' mathematical thinking. We present results of interactions between seven bilingual pre-service teachers and seven bilingual 5th-graders working on fractions. Using a theoretical framework focused on the use of one-on-one interviews, translanguaging, and responsive teaching, we identified two aspects of language in teaching mathematics in the bilingual classroom: (a) language as a hidden resource that bilingual teachers possess and (b) language as a resource not isolated from teaching mathematics. We argue that these two aspects are situated in, and integrated into, the practice of teaching and in this way should be part of the preparation of bilingual teachers.