You can programmatically apply text-align to the body element of the editor's IFRAME with the following code:
function OnClientLoad(editor, args)
editor.get_document().body.style.textAlign = "right";
<telerik:RadEditor ID="RadEditor1" runat="server" OnClientLoad="OnClientLoad"></telerik:RadEditor>
However, this will not make the editor's dialogs to be RTL aligned.
The full RTL support is logged in our ToDo list for Q3 with high priority. We will make RTL skin which will provide RTL support for the content area as well as for the editor's dialogs.
the Telerik team
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