At the transition from medieval to early modern Europe, vernaculars became more commonly used in writing in various domain, including administration and law. In the late Middle Ages, vernaculars seem to gain status and start to be regarded as suitable languages for recording municipal matters. But how did this vernacularisation progress in such documentary legal texts? When and why was Latin replaced by vernaculars? To answer these questions, this article presents diachronic quantitative analyses of the language choice in individual entries of two documentary legal sources: the first eight volumes of the Aberdeen Council Registers (1398–1511) and one volume of the Lübecker Niederstadtbuch (1430–1451). The comparative approach to these multilingual texts allows us to trace vernacularisation processes across two language contexts, uncovering similarities and differences between them. In the council registers from Aberdeen, Scots replaced Latin slowly and gradually – a process that took over a century and was far from finished by 1511. In the text from Lübeck, on the other hand, the shift from Latin to Low German was largely completed within about 30 years. In both cases, language practices of the scribes rather than top-down language policies seem to have driven this development. The scribes’ language choices will have been influenced by the use of vernaculars in other texts, including those in the domain of law, pragmatic considerations, and more general socioeconomic developments. By investigating vernacularisation processes in two sources, this article offers a first comparative account that allows for generalisations beyond individual language contexts and serves as the basis for further research in this area.