The protection against processing loops (i.e. the alreadyfiltered stuff) was
playing against us: the template plugin triggered a filter hooks run with the
very same ($page, $destpage) arguments pair that we use to identify a already
filtered page. Processing an included template could then mark the whole
translation page as already filtered, which prevented po_to_markup to be called
on the PO content.
This commit only runs the whole PO filter logic when our filter hook is run by
IkiWiki::render, which only happens when the full page needs to be filtered.