Thank you for writing.
Currently, RadRichTextBox for WinForms does not support RTL functionality. Such a request is already available in our PITS system. Feel free to add your vote for it here: http://www.telerik.com/support/pits.aspx#/details/Issue=8057
For your information our WPF and Silverlight suits support this functionality, so if you consider either of them as an option, feel free to check them out.
I hope that you find this information useful.
the Telerik team