It just came to my mind. You can also take English language pack. Edit it and select only resource strings responsible for admin panel. Every resource string in admin panel starts with "admin.". Then save that file, go to admin panel, edit your Norwegian language and import created language pack with English resource strings from admin panel. We can say that you will have two languages in one. Admin will be in English, front in Norwegian. Hope it will help you.
Thx for your replies. This works fine out of the box - your first answer made me realize that this is just a "dev problem". I just need to log in as different users when running from Visual Studio, so that the front-end user language won't affect the current backend language.
(Sidenote: A lot of translations in the Norwegian lang file are obviously machine-translated. We have imporoved some 100s of incorrect translations, and will probably continue to do so. When our translations are done, we'll be happy to share this as a continous open source file.)