We currently translate into Hebrew and try to move in rtl.
it is interesting that okhayat sharing his work on rtl in Arabic that he
make for a year already.

There's really nothing special I did for Arabic support. It's just the theme is RTL. Some things to watch our for:
1. First of all, setup your language, before doing the installation, and use it by changing /concrete/config/base.php line:
if (!defined('LOCALE')) {
define("LOCALE", 'en_US');
}
This will assure installing your localized data in example pages and dashboard.
2. Use utf8_unicode_ci for your MySQL DB
3. I did change the CSS for the C5 forms to be RTL, but that's really not necessary and I did it for an old version.
4. To avoid breaking C5's CSS, especially if you want to use direction:rtl in CSS, apply this only to your theme container, and not BODY tag.

Hope this helps a bit.