The preparation of bilingual prospective teachers (BPTs) in mathematics encompasses a challenge because many U.S.-born BPTs had limited opportunities to learn mathematics in their native language, in this case, Spanish. Consequently, BPTs feel ill-prepared to teach mathematics bilingually. This study describes the experiences of seven Latinx BPTs facing this challenge. A language socialization framework and the analysis of mathematics teaching practices through BPTs' beliefs, goals, and actions prior to and during a language shift reveal links to non- and re-negotiated Imagined Learning Communities. The relevance and necessity of teaching translingual identities and practices in mathematics education are discussed.