This allows mixing directionalities on the same page, which should be useful for embassies and sites like Commons with no official language (at least for content).
Suggested by @Amire80 at http://ee-flow.wmflabs.org/w/index.php?title=Topic:Sc660dyvwhbjemww&topic_showPostId=sc9tzzoy7xxtbn28#flow-post-sc9tzzoy7xxtbn28 .