I would not want to feed all the people who have told me that the two national languages of Belgium are French and “Flemish”, rather than Dutch.[*] Yet I am old enough to remember it being referred to by my elementary school teachers as ABN, Algemeen Beschaafd Nederlands — “Common Civilized [!] Dutch”. (By the time I entered junior high school, ABN gave way to the somewhat more politically correct Algemeen Nederlands — Common Dutch, or Standard Dutch if you like. The Van Dale dictionary — the Dutch equivalent of the OED or Merriam-Webster — refers to specific usages in Flanders as Zuidnederlands — Southern Dutch.
In terms of the written language, e.g. as used by the newspaper De Standaard which I have frequently quoted in my COVID19 updates about Belgium, the variant used in Flanders is no more different from that used in The Netherlands than Oxford and American English are, probably less. The spelling differences of the latter two — like defense vs. defence, amortize vs. amortise,… — are completely absent between Algemeen Nederlands and Zuidnederlands. There are vocabulary differences, just like there are between British and American English — “Zuidnederlands” contains a fair number of French loanwords and calques (loan translation) from French, unsurprisingly given the exposure to Belgium’s second official language. The linguistic term for a situation like Oxford vs. American English, or (written) Algemeen Nederlands vs. Zuidnederlands, is a “pluricentric language”. (English of course also has Canadian, Australian, and New Zealand varieties, although these are definitely part of cluster with Oxford/British English, distinct from American English.)
But what about the spoken language? Now here things get murkier.
At the local level, especially among older people in rural areas, local dialects used to be commonly spoken, and to an extent still are. These come in four dialect clusters:
- Brabants/Brabantian as spoken in the provinces of Brabant and Antwerp, particularly the Dutch-speaking minority in Brussels and the majority population of Antwerp. (It’s quite possible that the 2nd language there today is Moroccan Arabic, and the 3rd Yiddish.)
- Oostvlaams/East Flemish dialects as spoken in the province of Oostvlaanderen/East Flanders, e.g. in the city of Ghent
- Westvlaams/West Flemish dialects as spoken in the North Sea coastal province of Westvlaanderen/West Flanders, e.g., in towns like Brugge/Bruges, Oostende/Ostend, and Ieper/Ypres
- Limburgs/Limburgian, which is linguistically closer to Plattdeutsch/Low German, as spoken in the Easternmost province as well as in adjacent Dutch Limburg.
The border between Dutch and Plattdeutsch/Low German is a classic example of a dialect continuum: the dialects of Aachen (Germany), Maastricht (Dutch Limburg), and Hasselt (Belgian Limburg) are mutually intelligible to a surprising degree. In contrast, dialects from “Bachten de Kupe” (the area of West Flanders enclosed by the sea, the Ijzer/Yser river, and the French border) border on incomprehensible to speakers of Brabant or Limburg dialects, or to the Dutch.
What happens in such a situation in most countries is that the two parties switch to their Dachsprache, “roof language” or “umbrella language” — e.g., a Bavarian and somebody from the Ruhr area will speak standard German to each other. Moroccan and Syrian Arabic, for instance, have limited mutual intelligibility, so when people from these places meet, they will usually communicate in Modern Standard Arabic.
Likewise in Flanders, any type of official, formal speech is likely to be in what you could call “newsreader Dutch”, standard Dutch but without the stereotypical “Hollands” accent, and with occasional Belgicisms.
Informally, however, one often hears people speak a form of what Flemish linguists call “tussentaal” (in-between language, intermediate language), or sometimes “soapvlaams” (soap opera Flemish) or “Verkavelingsvlaams” (housing development Flemish): a kind-of “common denominator of dialects”. Tussentaal is not in significant use as a written language (except, e.g., when representing dialect speech in Dutch-language fiction, or for effect). And while it is definitely true that highly educated speakers often speak what amounts to Algemeen Zuidnederlands, tussentaal is not a basilect (language of low socioeconomic strata) — tussentaal is spoken to greater or lesser extent in all social classes.
An example of a universal feature of all Flemish dialects that you will hear in tussentaal is the archaic “gij” or “ge” (thou) as the all-purpose second person pronoun, instead of standard Dutch trio “jij” (singular familiar you), “U” (singular respectful you), or “jullie” (plural you). In many dialects, and often is tussentaal, one might hear “gijlie” for 2nd person plural, much like “y’all” in the American South.
So what is Tussentaal really? Is it a regiolect (i.e., a regional dialect), or a cluster of regiolects? (Tussentaal is not homogenous across Flanders.)
Or is it rather a koiné language? The term originates with Koiné Greek, the “common Greek” in which the Christian New Testament was written (as distinct from the literary Attic Greek which some of us still learned in High School). Koiné arose as a kind-of lingua franca dialect between different Greek dialects. The Wikipedia article [caveat lector] on modern koiné languages cites spoken Québecois French and Australian English as two modern examples. (The formal written languages, in both cases, are slight variants of standard French and Oxford English, respectively.) The concept of a koiné, besides coexistence with local dialects, does admit of a mild degree of regional (and socio-economic) variation, so that does seem to be the best fit.
[*] In fact, German has official status in Belgium as well, because of the German-speaking cantons of Eupen-Malmédy-St.-Vith, which were attached to Belgium as part of the post-WW I Versailles Treaty.