I've always wondered how you would get German to be the majority language [and the official language] in the Netherlands, the two languages are quite similar, after all, it should be easy, I think.
So your challenge, if you choose to accept it, is this:
Have German be the language spoken by the majority of people in the Netherlands and be the only national official language of the Netherlands by 1900 with no POD before 1400.
-There may be Dutch-speaking regions which have Dutch as an official language, but it cannot be a nation-wide official language.
-The Netherlands means all of the netherlands as of the present day.
-The Netherlands doesn't have to be independent [i.e. It can be part of Germany or some ATL equivelant such as a more unified HRE or a massive Prussia].