If a stream has no language metadata, or has the wrong metadata, this
field allows the user to override this on the command-line.
This also maps all specified languages on the command-line to ISO-639-2
tags as required by the MP4 muxer, so that ISO-639-1 tags specified on
the command-line do not cause a DCHECK to fail.
b/18613148
Change-Id: I473baeecbb3d388db5e06d080179ec6a332b4794