They are running Forgejo. The text being Dutch on the main page of https://code.overheid.nl/ is probably either because they haven’t provided any translations for the text, or maybe they even put the Dutch version in the template itself directly instead of storing it in whatever DB table Forgejo normally uses for the text.

I run a Forgejo instance too for my own use, but haven’t looked into how translation is set up as I haven’t had any need for changing any of the templates or texts that ship with Forgejo by default.