Thank you for writing.
By default, the controls in the Telerik UI for WinForms suite are localized in English. In our Code Library, customers share different localization providers and dictionaries. You can refer to the following link where you can search for the desired RichTextBoxLocalizationProvider
Otherwise, you can create your own provider following the approach in the referred help article. You can download an XML file that contains all the currently used strings from the Important
note in the article. Thus, you can include them in the RichTextBoxLocalizationProvider.
I hope this information helps. Should you have further questions I would be glad to help.