[Python] Add xbmc.getLangauge(int format)

This PR allows addon devs to specify the format of the returned language string.
Possible values are:
* 0: full language
* 1: two letter code
* 2: three letter code

I've taken the ConvertToTwoCharCode method from @pieh's repository.
Thanks @pieh! 

The feature request: http://forum.xbmc.org/showthread.php?tid=163023