The localization of RadRichTextBox concerns only the pop-ups that RadRichTextBox uses to apply formatting to the document and the messages that are shown in some cases.
The content of the document cannot be localized, as it is normally created after the application is run and can be changed afterwards. Furthermore, spans are created, merged and split on the fly, depending on the formatting of the document in a way the user cannot control.
What you can do is to create translated versions of your document and load the respective one when the current culture changes. In case the document will be mostly the same, you can use the same template and add merge fields to it. Again, when the current culture changes, you can assign different ItemsSource to the MailMergeDataSource in order to achieve your goal.
I hope this helps.
the Telerik team
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