Provide extended language selection for preferred audio/subtitle stream selection

This addresses http://trac.xbmc.org/ticket/14118. Right now the list of languages displayed for the "Preferred audio/subtitle stream language" settings depend upon the languages installed with XBMC. If a user (mostly on win32) decides to only install the english language, he won't be able to choose any other language for the preferred audio/subtitle stream language settings.
This change lets the user choose from the english names of the ISO 639-1 language codes independent of the installed XBMC languages. We could also use ISO 639-2 but there are lots of languages in there which don't make much sense IMO (e.g. "English, Old (ca.450-1100)" or "English, Middle (1100-1500)") which I doubt are used in any media files.