[Nyubis]

The language is Dutch, the collection of dialects spoken in the Flanders region is Flemish. If you're feeling charitable, you can refer to it as a language variant.

Here is the constitution of Belgium: https://www.senate.be/doc/const_nl.html Articles 2 and 3 define the Flemish (Vlaamse) community and region. Article 4 defines the language areas and specifically mentions Dutch (Nederlands), not Flemish.

See also: the Dutch Language Union (https://en.wikipedia.org/wiki/Dutch_Language_Union) being founded on a treaty between the Netherlands and Belgium.

For a language that's derived from Dutch but now its own proper language, try Afrikaans.