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.