Digital libraries have become valuable learning resources for information users. However, language barriers have greatly limited information access for many digital libraries, as users do not understand those languages. This study explored technical and operational challenges digital libraries faced in sustaining multilinguality. Using the multiple-case method, the study investigated two digital libraries that have sustained multilinguality for over a decade: the World Digital Library and the Digital Library of the Caribbean. On-site interviews were conducted at both digital libraries and the related documents were analyzed. The findings of the study showed that the two multilingual digital libraries faced many technical and operational challenges and employed various approaches to find solutions. A model of challenges and approaches in sustaining multilinguality was presented. As the first such case study, this research enriches the existing literature, and has theoretical, practical, and methodological implications for the research of multilingual digital libraries. The findings of the study provide useful guidelines and insights for the digital library community in sustaining multilingual services.