I'm still looking for an answer to this but I do have a little bit of an update for anyone else that runs into this.
Telerik provides the option to customize the dialog controls that are used for the various commands by setting the ExternalDialogsPath property on the RadEditor. You can extract the ascx files used for the dialogs from the Telerik.Web.UI assembly (or download them from various support posts) but these controls can not contain any .net code. When these dialog controls are loaded they are rendered using the ParseControl method. This method only renders HTML and does not do any compilation. Here is the MSDN documentation, http://msdn.microsoft.com/en-us/library/kz3ffe28.aspx, which states the following:
parameter contains a user control, such as you would find in an .ascx file. This string cannot contain any code, because the ParseControl
method never causes compilation."
Personally I think this is very poor design on the part of Telerik and causes confusion to those of us out there trying to use the product. Why would you use a .ascx file for something that can only be HTML? It causes us to make the assumption that if the files are .ascx files then we can use .NET code inside them. This is unfortunately not the case. And unfortunately this stuff doesn't seem to be documented ANYWHERE.
As it stands I'm still looking for a way to programmatically set the localization strings for a command dialog for the RadEditor. I suspect it simply can't be done. Which is unfortunately for a control that claims to be such a full feature control.