One solution to changing the direction of the interface when it is displayed in a right to left culture is to include right to left overrides for each stylesheet. For example, if you have 'main.css', you might create 'main-rtl.css' with right to left overrides for 'main.css' and include it when the current culture is right to left.

#5636 proposes a patch which makes stylesheet_tag() include a right to left override for each stylesheet, if one exists.

This is thee approach taken by the Drupal project.

I think this approach is also taken by MediaWiki.