Well, they want to be two separate countries. Other than a couple of small German-speaking areas, Belgium is split into a Dutch-speaking north and a French-speaking south. When they broke free of the Netherlands in 1830 they were united by religion, but obviously that's not as big a deal nowadays. Wanting those two distinct groups to govern themselves without interference from the other isn't in itself a ridiculous thing for a member of one of those groups to want, and the internal political borders within the country do already reflect it to a degree. However, as with most things, I would definitely be wary of it when it's the far-right wanting to do it