Diglossia

Diglossia is a type of bilingualism where one can speak two dialects of the same language.

Source: tatoeba (2754394)

By agreeing to integrate the "Tamazight language" into its educational system, the perfidious and calculating Algerian state wants to annihilate the Kabyle language by creating a diglossia which will in turn create serious learning problems and cross-comprehension issues. Because this "Tamazight language" is full of neologisms, in addition to being an absurd mixture of several Berber languages, that often have no meaning and has no connection with the sociolinguistic reality for all Berbers. In the end, they all lose out.

Source: tatoeba (9502746)

To begin with, of the two varieties involved in diglossia, the one serving (H)igh societal functions, unlike that reserved for (L)ow ones, is nobody's mother tongue: it is learned in later life largely by formal education, and is not used for ordinary conversation. Secondly, the diglossic contrast concerns widely divergent varieties, as opposed to stylistic contrasts which tend to be small-scale. Moreover, diglossia occurs within a single language, while bilingualism or multilingualism, involve far more divergent linguistic systems.

Source: wiktionary

2000, Joshua A. Fishman, Chapter 3: Bilingualism with and without diglossia; diglossia with and without bilingualism, Li Wei (editor), The Bilingualism Reader, Taylor & Francis (Routledge), page 81, It is the purpose of this chapter to relate these two research traditions to each other by tracing the interaction between their two major constructs: bilingualism (on the part of psychologists) and diglossia (on the part of sociologists).

Source: wiktionary