This paper forwards the notion that languages are an important resource for sustainable development for modern societies. Informed by theories from both sociolinguistics and language economics on the value of language skills, it is suggested that language(s) have different kinds of value in multilingual societies. Sociolinguists often emphasize the value languages have to their speakers and highlight the importance of social justice and equality between languages. Language economists argue that proficiencies in languages other than the dominant language are a form of human capital that can have a range of direct or indirect benefits. In language economics this is captured by the concepts of non-market and market value. Based on the example of the situation of languages in Australia, this paper argues that migration-induced multilingualism deserves more substantial support from both perspectives. Employing a theory-generating approach, this paper argues that multilingualism forms an important addition to Australia's human capital. For sustainable development, both on economic and social levels, it appears necessary to maintain and support languages in order to enable the country to cater for present and future linguistic needs. These linguistic needs include job market demands, but extend to other aspects of social participation and cohesion.