Add a method to get the primary audio language.
Add a method to get the primary audio language.

This duplicates the functionality of video.subtitle_language(), but is way
easier to read and understand.

norayr/unisubs,ofer43211/unisubs,norayr/unisubs,eloquence/unisubs,ReachingOut/unisubs,pculture/unisubs,wevoice/wesub,ujdhesa/unisubs,pculture/unisubs,eloquence/unisubs,norayr/unisubs,ofer43211/unisubs,ofer43211/unisubs,wevoice/wesub,pculture/unisubs,wevoice/wesub,ujdhesa/unisubs,wevoice/wesub,eloquence/unisubs,ReachingOut/unisubs,norayr/unisubs,ReachingOut/unisubs,ujdhesa/unisubs,eloquence/unisubs,ReachingOut/unisubs,ofer43211/unisubs,pculture/unisubs,ujdhesa/unisubs