Yes, we have build a site that was meant to be used in arabic language (and sooner or later will be). Although it is possible, there are some inconsistencies in the backend.
1) you will have to write manual checks for switching reading direction. this will be dependend on the logged in user settings. this works good in many situations (tinymce), but somehow it doesn't work an all fields.
2) its not possible to switch directionality for every item in the admin UI. Serveral field descriptions are displayed at the top left corner of a text input field, although writing inside this textbox works RTL. you will have to make some tradeoffs according work amount <-> complete arabic UI
At frontend level everything (except metadata) can be achieved by css files, that can be comfortably loaded according to active language. Metadata stuff is a matter of template design, this works well if you know how to use translatable and i18n extensions.
If i use 'he_IL' (hebrew) or 'fr_FR' (french) everythings works as expected. But NOT 'ar_SA' (arabic - saudi arabia).
The arabic lang is missing in the "language" pulldown on the left area of cms AND it is missing inside the translation tab of a page...
Did you also set i18n? Don't know if this may have any effect, but I always used them together and SiteTree also (somehow) uses i18n.
// Set the site locale and allowed languages
$allowed_locales = array(
'de_DE' => array('German', 'Deutsch'),
'en_US' => array('English', 'English'),
'ar_EG' => array('Arabic', 'العربية')
i18n::$common_locales = $allowed_locales;
// Make Pages translateable into other languages
Translatable::set_allowed_locales(array('en_US', 'de_DE', 'ar_EG'));