Education that contributes to sustainable development embraces social learning that breaks down boundaries between schools and communities. Such learning is epistemically inclusive. This article argues that epistemic inclusion in multilingual societies is contingent on using students’ familiar languages within basic education. Across subSaharan Africa, secondary school students learn in a dominant language of learning and teaching (LoLT), which for the majority is an unfamiliar language. Additive multilingual education that develops learners’ familiar language alongside introducing a more dominant language enhances epistemic access to the specified curriculum. It also opens up the possibility for social learning that brings curriculum knowledge into conversation with community knowledges. Drawing on examples from the literature and authors’ own research in Rwanda and Tanzania, the article argues that additive multilingual education is necessary for education that contributes to sustainable development.