To change the language of a WordPress site, just update the WPLANG constant in your wp-config.php file:

define ('WPLANG', 'ar'); //changes the language of the site to Arabic

But this will change the language of both of your Front End and Back End. Separating these two was not possible if it weren't for this WP plugin by Louay(same author of The RTLer): Admin Locale.

Install the plugin and:

Just go to Settings » General and choose your language.

Note: Of course you'll need to install a localized version of WordPress so you can get the language files. Arabic WordPress can be downloaded from http://ar.wordpress.org

Enjoy =)