Read locale from .po if it has Language header

Read locale from .po if it has Language header

When a .po file was written out it would write the "Language"
header, but it wouldn't subsequently read that header back in. This
change makes that symmetric so you can write out a file and get back the
same Catalog by reading it in again.

Additionally other values like charset get overridden when the header is
read, so codify the behavior that the read "Language" header gets set as
the locale of the Catalog object.