Localization of RadRichTextBox is already fully supported, although this is not well documented for now. You could refer to the following article about localizing RadControls in general. As RadRichTextBox doesn't have detailed description of its resource keys, I have prepared you sample resource file (find attached) that you can use for your translation. If you are interested in localization of the RadRichTextBoxRibbonUI you should know that this control is fully customizable through XAML, so you can put your localized strings directly in there.

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Thank you for your question.

Indeed the localization strings for LeftToRight and RightToLeft are missing in the attached file. This is so because Boby's post is for an older version of the controls.

The localization keys you have asked about are:
Documents_ParagraphPropertiesDialog_Direction_LeftToRight
Documents_ParagraphPropertiesDialog_Direction_RightToLeft. 

In this knowledge base article you could find all localization strings that RadRichTextBox and its default UI use. You can also refer to this article which describes how the localization of RadRichTextBox can be done.

I hope this information helps. Please, do not hesitate to contact us if you need further assistance.