This is the closest you can get. In Norway, mellomfag is part of the BA degree, which consists of "grunnfag" (foundation), "mellomfag" (intermediate) and "storfag" (major)levels. "Hovedfag" is something more than a Masters, it is somewhere inbetween a Masters and a PhD.

The University of Oslo web page in English translates these as:
mellomfag = intermediate course
hovedfag = main subject
See: http://www.uio.no/english/academics/admission/academicsystem...
I'm not particularly happy with these translations, but as the educational system is different, there isn't much you can do.

My personal suggestion would be:
If this is for a translation and not for personal information, leave the Norwegian names and put in parenthesis an approximate translation as the one suggested above: Mellomfag (BA) and Hovedfag (Masters). I've already seen it like this in Norwegian people's CVs. If you want and can afford to be more explicit, say Mellomfag (BA, Minor in whatever it is you have) and Hovedfag (Masters with major in....). But I wouldn't opt for this, it's too long.

The University of York in the UK which has an agreement with a Norwegian educational institution leaves the terms untranslated (Mellomfag studies, Hovedfag degree), but this is addressed to people who know what it is about. See: http://www.york.ac.uk/inst/nsc/nsch.htm

Hope this helps