how can I install a Norwegian spell checker in Mail?

Apple doesn't provide spell checking for all the languages supported by OSX.

Install CocoAspell and download one or both of the Norwegian dictionaries:

http://m10lmac.blogspot.com/2007/01/spell-checking-in-other-languages.html

and/or

<http://cocoaspell.leuski.net/>

It has a Norwegian dictionary available.

An updated version of the link provided by Klaus1

http://m10lmac.blogspot.com/2011/06/extra-spell-checking-dictionaries-for.html

Also see

http://home.no/stavekontroll/mac.html

Thanks Tom! I should have checked that first, but haven't needed it for about 2 years!