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:
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.
the Telerik team