ENThe paper describes the author’s contribution to research project of Vilnius University Library (hereafter: VUL) whose aim is to create a digital database of manuscript court books of the Grand Duchy of Lithuania (hereafter: GDL) from the VUL collection. Circumstances of creating of the analysed documents are closely related to the features of the cultural and linguistic situation in the GDL which has been remarkable for its polyethnicity since olden days.The article presents observations concerning the chronology and distinctive features of the usage of Slavic languages in GDL legal texts that reflect the social and political transformations in the 16th–18th centuries.