ENThe main problem, which is placed in the article: why the Lithuanian language has not received the official status in the State, which was called the of the Grand Duchy of Lithuania (GDL)? And this happened at a time when the beginning of the 16th C. have survived the first written examples of fixation of the Lithuanian language, and from 1547 there existed a rich tradition of the Lithuanian printing (predominantly in the Duke of Prussia). The author sees a possible answer to the recognition that in a multiethnic and multicultural society of the GDL in 15th–17th C. there was a language situation «a lot of diglossias». In this situation, for each language have been clearly set out certain public functions. In contrast to bilingualism in the socio-linguistic phenomenon of diglossia one who says, evaluates its idiom on a scale of «solemn – everyday». At the same time the public sphere is not used two language systems (formal and folk).