making a book in Pages

There is no Dutch Pages community. However, if you click on one of the links your posted, say…

Create an EPUB book in Pages - Apple Support

then change the language in the bottom right to Nederland, you get…

Een EPUB-boek aanmaken in Pages - Apple Support (NL)

Apple does not yet provide a Dutch language community. In the lower-right of this page, you may see the United States, and clicking that country name brings up a new page showing community language support other than English.

There is a dedicated Pages community and other reading:

Intro to creating a book in Pages on Mac - Apple Support

Use advanced book creation options in Pages - Apple Support

Publish your book with Pages - Apple Support

Create an EPUB book in Pages - Apple Support

That link that you provided indicated that there was a Nederland support community site, but there currently is not one shown by Apple, and although there is one for Belgique (Belgium), I am unaware that it would pass for proper Nederland communication. 😉

See if this thread helps: dutch - Apple Community

Informatie over het aanmaken van een boek in Pages op de Mac - Apple Support (NL)

How is that possible when I don't see Nederlands as a supported language? What am I missing?

Choose your country or region - Apple Community

Okie doke. 😎

Just to add, all the links you posted are available in Dutch.

Today, I see a much more robust list of european languages (including Nederland) than I did yesterday before my post.

FWIW, not all the pages include all the languages for whatever reason.