A conception of linguistic justice refers to a set of moral principles that can provide guidance about a society’s language policies. By the lights of a recently prominent strand of liberal political theory, such a conception will be legitimate if and only if it meets certain standards of public justification. Amongst other things, this means that it must be arrived at by way of a legitimate procedure and its principles must be ones that no one could reasonably reject. This chapter explores the potential of this ideal for mobile and multilingual societies in Europe by developing three arguments. First, we argue that in order to be legitimate, a conception of linguistic justice must be justifiable to a relevant constituency and that, in EU member states, this includes mobile Europeans as well as the citizens of member states. Second, we argue that, in circumstances of linguistic diversity, the requirements of public justification generate a presumption in favour of multilingual deliberative procedures. Third, we argue that when selecting principles of justice we ought to prioritise the interests of the least advantaged, and that doing so will often mean that only a multilingual language regime is acceptable.