Manual:$wgBetterDirectionality

Better directionality (left-to-right, right-to-left) support, most importantly being to able to use an LTR interface on an RTL wiki and vice versa.

Details

 * In rev 69185 (1.17), the page lang and dir is in the user language, and a div is added to bodyContent with lang and dir attributes based on the site content language. This means basic distinction between the interface and site content language, instead of marking everything in the site content language (old, default behavior).
 * In ResourceLoader, the directionality was set according to the user language, but this was reverted in rev 81622.
 * In rev 90264 (1.19), it was re-added under $wgBetterDirectionality = true, and in follow-up commits the implementation was further improved.

So far it has been enabled on Translatewiki.net and it will be enabled by default soon.