I don't know. I was thinking of adding Brussels to the freestate, but Brussels was a Fench speaking city; at least the upper and probably the middle class considred themselves that way. So in the end I decided to add it to the french areas. Still i could see it different in a negotiated Talleyrand division. Although to be fair, I don't think the Netherlands would care that much. They are losing a big part of their country anyway and wether Brussels was French, Antwerpian, Dutch or Prussian wouldn't realy matter for them.
Personally I prefer a Flanders-Walloon split between the Netherlands and France, which I think would be better for the Netherlands, the Flemish, France and probably even the Walloons (they will be a very important part of a much stronger country). I am perfectly willing to give up the Walloon industrial base for a lack of linguistic problems. Also the Catholic-Protestant divide will not be as unbalanced as when Wallonia would be part of the Netherlands. If Flanders would be part of the Netherlands, it would be 40% protestant/60% catholic. If the Walloon areas were included the catholics would totaly dominate the country, which would probably cause problems in the traditional protestant Netherlands (OTL was 60% protestant/40% catholic).
brussels at the time was not yet uniformly french, the earlier the POD the more likely the Dutch language can be maintained. If a United Kingdom of the Netherlands were to remain the most important party to not hate Dutch would be the Church. The Catholic Church saw Dutch as a heretical language of the equally heretical protestant faith, IF Willem I would allow religious schools to be paid for by the government this could prevent a lot of problems from arising.
Perhaps the law could state something like:
"Scholen in het Verenigd Koninkrijk der Nederlanden zullen onderwijs geven gebaseerd op Protestants-Christelijke normen en waarden en religieuze opzattingen danwel in Katholiek-Christelijke normen en waarden en religieuze opvattingen. Aldus zonder haatzaaing jegens eenander."
(in English: "Schools in the United Kingdom of the Netherlands will teach based upon Protestant-Christian norms and values and religious beliefs or in Catholic-Christian norms and values and religious beliefs. This without causing hatred against one another.")
This in conjunction with the removal of the ban on a Catholic becoming the Monarch and perhaps allowing the provinces to determine if they are Catholic or Protestant. Add to that the more liberal constitution Thorbecke has in OTL created and the United Kingdom of the Netherlands may actually survive.
Now if you guys want me to go into more about the various versions of the Dutch constitutions I can do so, but then I will need to go to the library and do a bit more research.
Then as to the Taalbesluit of 1823, I suggest Willem is to be more lenient, allowing French to be used still, but stimulating the populace to learn Dutch via tax benefits and have the schools teach Dutch as the primary language and French as the first foreign language.
Getting the frenchified Flemish notables to start speaking Dutch get them to be invited to the Royal Court and allow the King to grant noble titles, but have the Royal Court use DUTCH, not French, forcing those nobles to learn Dutch to get those nice titles. Ofcourse those notables would be granted an official translator so they can understand what's being said, while at the same time making sure they are outnumbered by the Dutch speakers.
I think this may help to enhance the prestige of the Dutch language and as such help the Dutch language to advance.