Translations: Run "make -C po update-po"

Translations: Run "make -C po update-po"

In the past this wasn't done before releases; the Git repository
just contained the files from the Translation Project. But this
way it is clearer when comparing release tarballs against the
Git repository. In future releases this might no longer be necessary
within a stable branch as the .po files won't change so easily anymore
when creating a tarball.