In many countries children need to become proficient in both their home language (L1) and an international language, such as English (L2). Governments face trade-offs in how to prioritize these two objectives. We provide empirical evidence on cross-linguistic transfer between L1 and L2, using results of two randomized evaluations of Structured Pedagogy Programs implemented in South Africa. The programs had the same design, implementing organization, and duration. The key difference is that one program targeted the teaching of reading in L1, while the other targeted L2. We find that both interventions had positive effects on the languages they targeted. The L1 intervention also had a positive effect on L2 reading proficiency. In contrast, the L2 intervention had a negative effect on L1 outcomes, for the lower-performing students. These results are consistent with the Simple View of Reading and suggest that decoding skills are best learned in L1. It is thus cost-effective to prioritize learning to read in L1, as well as supporting teachers in this subject, even if becoming proficient in L2 is also regarded as an important policy objective.