Sentence:  To begin with, of the two varieties involved in <WSD>diglossia</WSD>, 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, <WSD>diglossia</WSD> occurs within a single language, while bilingualism or multilingualism, involve far more divergent linguistic systems. 
Meaning of the word marked with <WSD> tags:  The coexistence in a given population of two closely related native languages or dialects, one of which is regarded as more prestigious than the other; the similar coexistence of two unrelated languages.