I cannot change my default language to Norwegian in the Control panel. The language (nb) is not listed as an option. In the directory webtrees/resources/lang the Norwegian language is listed in the file nb and the contents of the file looks correct.
Why is the language not listed as an option?
Have you checked in the control panel (Modules > Languages) whether the respective module is activated? Or do you mean the language doesn't appear there? If it isn't activated, it's not available as a language option for users of your site.
webtrees 2.0.15 at
Vesta custom modules (Classic Look & Feel, Gov4Webtrees, Shared Places, Extended Relationships) available at