I know that nb is already 60 % translated in MediaWiki, but is there a reason not to use translations from another closely related language, rather than English, when no translation is available? It's always nice when we don't recur to English.
According to http://www.let.rug.nl/gooskens/pdf/publ_JMMD_2007.pdf , for instance, written Swedish is 88 % intelligible for Norwegian speakers. Swedish is around 80 % translated as of now.
We could also do some reciprocal fallbacks, but we should ideally have intelligibility studies for any fallback we add.
I hadn't thought about there being no other fallbacks. Norwegians understand (written) Swedish and Danish just fine, but in my mind I think it makes more sense to have missing translations just fall back to English, as that signifies that there is no translation, while on the other hand if they were to fall back to Swedish, I can imagine some complaints and users questioning if there's a bug that makes stuff appear in Swedish rather than Norwegian (since it's not obvious why it is in Swedish). nb and nn should have reciprocal fallback though, I can't imagine any complaints about that.