When normalizing language tags to their shortest form, we need to
split off subtags and treat them as separate components that do not
get changed.
Furthermore, MP4 can only store language tags without subtags. So
only store the main language in MP4 output.
Fixes b/27533973
Change-Id: I96049e274aae9841e321c53039ef6464a2e61574
Additionally, for XML schema correctness, convert ISO-639-2 language
tags (three-letter codes) to ISO-639-1 tags (two-letter codes) when
possible. This follows BCP-47, which says to always use the shortest
language tag when there are multiple possibilities.
b/18613148
Change-Id: I120fb7b42ac4da5feb4ca046bba93d9e58acd7a9
Joey Parrish