BuddyPress.org

Do I need to translate WPMU first ?

Look in the link that DJPaul provided. There is a WordPress translation of Arabic already. I’m not sure what additional work you’d have to do for WPMU. Most likely translation of some of the admin pages. But, I’m not a localization expert.

What will be different because Arabic is RTL language ?

As long as the themes that Arabic users are using properly reference some type of right to left CSS file, then this should not be an issue. Look at rtl.css to see how that works.

Also, look at this relevant BuddyPress thread: https://buddypress.org/forums/topic.php?id=245