The reason it dosen't appear selectable is simply because the game dosen't have it in the UI selection list. If you open up assest1.pk3, look in /ui/setup.menu and /ui/jamp/setup.menu.
In /ui/jamp/setup.menu, you'll see:
I'm not sure if you need to modify the game source for it though, as I personally don't really need to add any languages :X
Looking back, thats for voice. The cvar in question for language though, is
"se_language", atleast, according to the UI files.