During the long process of unmaking of the Ottoman Empire, international congresses and conferences shaped the laws on property after empire in the new successor states. One of the important legal mechanisms established in this period were bilateral commissions, each comprising members from the Ottoman Empire and the relevant successor state. The task of such commissions was to negotiate solutions and indemnities for certain legal types of property, but due to many factors, including the lack of guidelines and expectations from their work, it was almost certain that they would not function successfully, let alone resolve any of the extremely complex tasks they faced. In this presentation, I examine the documents of one such mixed commission, established in 1878 to arrange the future of waqf and Ottoman state property in Serbia. I focus on the concept of legal untranslatability (inspired by Cassin 2018 and Salaymeh 2021): the claim that a certain legal term does not have a counterpart in the target legal language. I argue that Serbian commission members claimed untranslatability not only in case of specific Ottoman-context terms, such as waqf, but also with regard to the entire Ottoman culture of property documentation. However, rather than simply a diplomatic strategy created for the needs of the moment, this claim was an outcome of a longer process of social and cultural reorientation in interimperial Serbia. The argument of untranslatability in that sense did not serve (only) the current diplomatic purpose, but it enabled Serbia to position itself in alignment with the European states and their legal usage, against the purported Ottoman legal “backwardness.” The presentation contributes to the view of diplomacy as a field in which uneven discourses of power are negotiated, and which is as such not separable from other political and legal fields of trans- and interimperial negotiation.