so the likely option are : the name of Pays de par-deçà (advantage it's the current name of the realm, it show their particulairism against the French, this name means that their land are beyond the french so are not french even if they are culturally close of the french, so it will also pleased the french speaking province of this realm, disavantage it's a french term, and it was the same name that was used for the netherlands in begining (before it was transformed in Pays-bas netherlands) so Dutch could use this name to claim these land.
another name like Belgicae, Batavorum, ... (advangtage it a new name that claim their particuliarism and would help to the realm to create a new identity in the long term, disavantage these name are anachronic, in this period i don't see a king use this kind of term fallen into obsolescence and would not be liked by the common people, weakening the foundations of the country in the short term).
you could also use the term of X province, but it will would imply an idea of confederation at least in begining.