Which CMS is the best to develop a multilingual website, that can be operated in English and Arabic simultaneously i.e RTL / LTR support ?

For me, Joomla is the best web tool i had try so far... you can install extensions, templates, modules very easy and even you can make your own templates using the artisteer and sothink logo maker..

I would say Joomla would be better for a multilingual site, you can easily create articles/post in a different language and assign to different language menu item and with that, you can create a access level for that too.  

Basm allah alrahman alrahim

wordpress but joomla and drupal are so strong