To reproduce: use the code snippet below. Please refer to the attached screenshot.

public Form1()
{
    RichTextBoxLocalizationProvider.CurrentProvider = new MyRichTextBoxLocalizationProvider();
    InitializeComponent();
}

public class MyRichTextBoxLocalizationProvider : RichTextBoxLocalizationProvider
{
    public override string GetLocalizedString(string id)
    { 

        return base.GetLocalizedString(id);
    }
}

Workaround: 
Modify the XML file containing all the strings RichTextBoxLocalizationProvider.CurrentProvider = RichTextBoxLocalizationProvider.FromFile(@"C:\RichTextBoxStrings.xml");. You can download a XML file that contains all the currently used strings from here: http://www.telerik.com/docs/default-source/ui-for-winforms/richtextboxstrings.zip?sfvrsn=2