The biggest problem is you cannot set the default to a language where there are any missing keys in the resource files or it will throw null reference exceptions. So the first thing is you would have to update all the *.nl.resx files (In the App_GlobalResources folder) and add translations for any missing keys as compared to the English file which is just named *.resx
Note that our Dutch resource files are just named *.nl.resx not *.nl-NL.resx, so your changes would need ot be like auto:nl
Also you need to understand what the "auto" does, it says to use the browser language preference, so if the browser language preference is still set to en, then it will show English resources instead of the default. So with auto, it will will always use the browser preference as long as a resource exists for the language specified by the browser preference, otherwise it will use the default. To force it to always use Dutch, you would have to remove the auto:nl and just use nl. But again, the problem is there are missing keys in the Dutch resource files so if you force it to Dutch you may get null reference erros whenever it tries to use a missing key. The errors may only happen on some pages.
Hope it helps,