SS looks for the current user's locale first, failing that it falls back on the site default. I'll bet the user you're logged in as was created when your site was set to en_US as the default. Ensure your user account's language is set to nl_NL or try logging out and viewing your site.
Hmm, I'm at a loss then. The New Zealanders should be coming online soon, I'm sure one of them will be able to assist.
The wiki is the sole source of documentation at this time (aside from sifting through the code yourself). When you get this sorted out please be sure to update it or let me know and I'll be happy to do it.