This article examines aspects of linguistic behaviour, attitudes and professional practices amongst a group of 47 "expert users" who are translators or interpreters for one, two or three of the following languages: Bosnian, Croatian and Serbian. The official terms for these languages in the respective successor states of Socialist Federal Republic of Yugoslavia and elsewhere reflect not only historic but also popular designations. Incumbent on translators and interpreters are the professional regulatory norms that require practitioners to follow these designations. An overview of the re-codification of the three languages is provided, followed by a discussion of models that account for how speakers and text-producers negotiate verbal and written interactions with speakers or text of different language varieties. Data are elicited on the following: informants' reported behaviour in professional and non-professional situations; unanticipated differences in the language for which an assignment was accepted and its actual form; attitudes on assignments with unofficial or unclear language designations; others' assumptions of informants' native speaker competency and ethnicity; and attitudes towards the distinctness of the three languages. Accommodation to clients' language varieties is reported by half of all informants, and those with multiple accreditations report converging to others' language varieties more so than those with accreditation in one language only. Metalinguistic talk, with or without accommodation, is also a common practice in the initial stages of interpreted interactions or the negotiation of translation assignments. The reported behaviour and practices of translators and interpreters are likely to be indicative of "lay speakers'" and marketplace requirements, and therefore reflective of actual language use amongst users of these three languages when interacting with one another.