u just install one folder
and from the site manager you will switch to Arabic language
my advice don't do that online
first try to do that in local server
or at least make full backup from your site first

for more information see these sites

http://joomlarab.net/
http://forum.joomla.org/viewforum.php?f=17&sid=031a8fb43f4552c7c69b0fb38...
https://www.joomlaeyes.com
http://www.joomla-ar.com