My understanding is that Mac OS X language support is modular - adding support for a new language involves nothing more than adding a new language project to the relevant application(s) (this can be easily done in Xcode - I don't know how difficult it is to make one outside of Xcode using an existing one as a template, but I'm sure it's possible).
Therefore, yes, you can add more languages, if you can find pre-built the needed language projects, or can do the translation yourself.