[gpvos]

The prestige dialect in Flanders, as spoken on the main news and public affairs programmes on public television (I don't know about the schools and courts), is already very recognizably different from that in the Netherlands, with its soft "g", slightly different vowels and somewhat different vocabulary. But it is losing terrain fast: the commercial TV channels, and entertainment programmes on public TV, are mostly using a hybrid between Flemish dialects and the (former?) prestige dialect that rather leans towards the Brabant dialect ("tussentaal"). Netherlands Dutch and Flemish Dutch, while still mutually understandable without much effort, have started to diverge more in the last decades. Although I haven't noticed people replying in English yet.

[CRConrad]

But then Flemish often claims to be a separate language from — as opposed to a dialect of — Dutch, doesn't it?

[gpvos]

Some people do. Linguists on the whole don't, and it will be a long while before they do. Literature is still completely shared, and we can read each other's newspapers with zero problems.

It's good to distinguish between Flemish as the group of dialects spoken in roughly the provinces of East- and West-Flanders, and Flemish as the broad name for everything that's not quite the standard language in the larger region of Flanders, which is the Dutch-speaking half of Belgium and includes three more provinces. I'm not sure which of those two those people refer to.