Hello,
I'm sorry for inconvenience, but you will still need to rename your Dutch resource file to be CMS.resx if you want to use CMSDefaultUICulture option. I believe that you've gone through that, but for reference I'm showing that this approach is described in our Developer's Guide:
Multilingual support and cultures overview.
Just for my curiosity, could you please kindly send me some article where it's described it shouldn't work like that? Thank you in advance.
Regarding the logon page translation, you can do that in code-behind of the logon page
~\CMSPages\logon.aspx.cs in Page_Load method like this:
CMS.GlobalHelper.CultureHelper.SetPreferredUICulture("nl-nl");
this.SetCulture();
Hope it's fine with you.
Best regards
Ondrej Vasil