[BjoernKW]

That's certainly a good explanation.

Still, for instance I wasn't able to find anything for either the French or the German authorities in their respective local languages.

For Dutch government services on the other hand there's the Rijkshuisstijl styleguide: https://www.rijkshuisstijl.nl/

(I just didn't find it at first because I searched for something along the lines of "nederlands overheid" rather than the official term "rijksoverheid").