I am not so sure. The North East of the Netherlands also spoke Low Saxon. Besides that, East-Frisia was closely connected to the Netherlands in the 17th and part of the 18th century it had been effectively a Dutch vasal state (the administrative language might even have been Dutch, not sure) and at the time of the kingdom of Holland it was even part of the Netherlands. Luxemburg and Limburg were Dutch and part of the German Confederation, but Germany never declared war over it, like they did with Schleswig-Holstein. Good relations with the Netherlands were more important than a small piece of land. Especialy considering that East-Frisia was significantly less important than Luxemburg. And that is even assuming that East-Frisia would become part of the German confederation, which I doubt. Luxemburg was only made part of it because of the compromise between Prussia and the Netherlands. Other parts that used to be German (or at least part of the HRE) were not, like all of Belgium. So I suspect that East-Frisia would become an integral part of the Netherlands, not just a personal union.