The distinction between a national language and an official language is made clear by Fasold:
"Official languages are in place primarily for nationalist reasons, those concerned with the day to day practical tasks of governing. The nationalist function is concerned with nationalist motivations, the unifying and separatist functions, the link with the glorious past, and authenticity."
Fasold goes on to propose that a nationalist function requires six sociolinguistic attributes:
(1) The language is a symbol of national unity for many people.
(2) It is widely used for some everyday, unofficial purposes, but not necessarily a home language.
(3) A sizable proportion of the population speaks the language fluently.
(4) There is no major alternative nationalist language, even though other languages may fulfill the group function on the regional level.
(5) The language must be seen as authentic, as "good enough."
(6) The language must be seen as a link with the glorious past.
Yet, as Fasold notes, if the language in question fails to meet 1 of the 6 requirements, it will probably fail in the nationalist function.
Fasold, Ralph. The Sociolinguistics of Society (Oxford, U.K.: Blackwell, 1987), p. 73.
Vachek’s Definition of Written Language
Vachek’s Functionalist Approach to Written Language
Difficulties in Freeing Semantics from Pragmatics
Distinguishing Pragmatics from Semantics
Pragmatic Accounts of Communication