Kitchen language is an elitist colloquial phrase for any language or dialect understood as inferior to an official language. I recommend avoiding the term unless you find necessity in its usage because it is, except perhaps in academic usage, often discriminatory by class, race, and/or ethnicity. There are several sources which can guide to a more full understanding of the phrase's origin, usage, and subtle meanings.
One usage of kitchen language is found in an analysis of the Buryat people, a Mongolic ethnic group in Siberia, and their place under Russian hegemony. The quote below demonstrates the social devaluing of their language through the application of the term.
When Buryat is called a “kitchen language,” it is devalued for functional domains outside the home, its use as a literary standard downplayed or dismissed. 1
Another example of kitchen language, taken from an examination of the development of Central European language, demonstrates the inherently elitist connotations of the phrase.
This dichotomy also obtained, and to a degree still does, between the educated and empowered who speak 'properly in the language' of a state capital and the socially removed 'riff-raff' talking in 'a broken language,' 'kitchen language' in the very same capital, or worst of all, in a 'dialect,' when the subaltern group's speech is spatially farther removed from that of the elite's. 2
The final example reveals the negatively racial origins of the term. The below quote is from a major textbook on the history and effects of colonialism.
Afrikaans has been called a kitchen language due to the frequent racial mixing among its speakers and the intimate household contact between white settlers, Cape Muslims, the Khoi, and others. 3
 Graber, Kathryn. “The Kitchen, the Cat, and the Table: Domestic Affairs in Minority-Language Politics.” Journal of Linguistic Anthropology, American Anthropological Association, 2017, www.academia.edu/35010298/The_Kitchen_the_Cat_and_the_Table_Domestic_Affairs_in_Minority-Language_Politics.
 Kamusella, Tomasz. “Creating Languages in Central Europe During the Last Millennium.” Google Books, Palgrave Mamillan, 2015, books.google.com/books?id=BqLtBQAAQBAJ&pg=PT23
 Sonnenburg, Penny M. “Colonialism: An International, Social, Cultural, and Political Encyclopedia, Volume 1.” Edited by Melvin E Page, Google Books, ABC-CLIO, 2003, books.google.com/books?id=qFTHBoRvQbsC&pg=PA7.