When available in multiple languages, educational apps can deliver the same mathematics instruction in the child’s first language or different language of instruction. This pilot study examined the feasibility of a maths app intervention compared to standard mathematical practice with 61 children aged 5–6years attending a bilingual immersion school in Brazil. The apps were delivered in either the child’s first (L1, Brazilian Portuguese) or second (L2, English) language, while app content, time on task, and school setting were consistent across the two treatment groups. Time on task in the comparison group was less. After 10 weeks, results showed children made significant mathematical learning gains with the apps, for both languages of instruction, compared to standard practice: the finding is discussed in relation to the threat of time spent on task. Children using the apps in Brazilian Portuguese (L1) completed more app topics than children using the apps in English (L2) and proficiency in language of instruction correlated positively with app progress. As children’s L1 was significantly stronger than their L2 proficiency, this suggests the apps were most effectively implemented in their L1. This study advances the conjoining of bilingual educational theories to educational apps in a controlled, real-world, bilingual immersion setting and highlights important directions for improving app-based mathematics instruction for bilingual children.