* glom/libglom/translations_po.[h|cc]: write_translations_to_po_file():
Add an optional locale_name parameter. We cannot discover this inside
the function because we only use iso-codes in glom, not libglom, and
I would prefer not to move that static data into libglom.
* glom/mode_design/translation/window_translations.cc:
on_button_export():Pass the extra locale_name parameter.
* tests/translations_po/data/test.po: Resave.

This makes the written .po file pass validation by msgfmt -c on the
command line.