If you're using the RTM edition of SharePoint 2013 Foundation, you could install the language pack for Dutch here : http://www.microsoft.com/en-us/download/details.aspx?id=35492 (make sure you pick the dutch file)
There should be downloadable as well for Standard / Enterprise but I didn't find them.
Once the installation will be finished on all your server (except your database one obviously), run the configuration wizard (again, all server).
From your site collection settings, you'll be able to specify additional available language ( click on “language settings” under site administration section) and then do the switch, per user, on the site interface (select display language).
You downloaded the packages for the Preview edition which was available from Summer 2012 but is now somehow deprecated so the issue you're seing might be a side effect of running a package for a CTP / Preview release on RTM binaries.