This paper studies the effect of language proficiency on Math achievement for ten-year-old second-generation immigrant children in the Italian primary school. Through an IV strategy that exploits the heterogeneity in birthdates and the variation in linguistic distances, we find that these children face a trade-off between learning Italian and learning Math. We show that this outcome is led by children with a non-Romance linguistic background, which does not allow to benefit from similarities to Italian. As suggested by the linguistics literature, we provide supportive evidence that this result arises from the existence of proficiency thresholds that have to be crossed in order to understand the classes. We confirm that the trade-off arises when proficiency is below the threshold that is commonly adopted to indicate a sufficient command of the language. Finally, we introduce an intuitive model of the learning process that provides theoretical foundations to our results and a unified framework to interpret the mixed findings in the literature.