Using the concepts of practices and representations, this paper seeks to find a way in order to analyse precise choices of language for interpersonal communication in a plurilingual environment whereas these choices might seem singular or conformist. The field of research is the Habsburg monarchy, especially Bohemia at the end of the 18th century. The important factors impacting the phenomenon of plurilinguism in this area are first presented. Then the precise occurrence of various languages is determined on the basis of the book production, books collections and what we know about the reading practices of the inhabitants. A first sketch of the main characteristics of different languages can be drawn (French is mainly used by aristocrats, sciences are more often read in German than in French, Latin is still used for law literature, Czech has become mainly an oral and popular language, etc.). Contemporary testimonies provide us with other, sometimes conflicted, representations about languages (French is depreciated, "mother tongue" becomes an exclusive value, the use of some languages is systematically associated with a social group). This paper then focuses on an individual for whom a good documentation enables us to define his personal linguistic abilities, the representations he shared and his main linguistic practices, especially in his diplomatic offices: Franz Anton von Hartig (1758-1797). For his case, we analyse strategies he implements in interpersonal communicative situations in order to free himself from the dominant representations and practices and confer an original meaning to his language choices.