There are some special characters in the Polish alphabet, which are inserted with key combinations - AltGr + E should insert "ę", Alt Gr + L should insert "ł". These combinations are overridden in RichTextBox because are triggering default commands in Silverlight.
 Workaround: Attach to the PreviewEditorKeyDown event:
  this.radRichTextBox.PreviewEditorKeyDown += (sender, args) =>
     {
         if (Keyboard.Modifiers.HasFlag(ModifierKeys.Alt) && Keyboard.Modifiers.HasFlag(ModifierKeys.Control) && args.Key == Key.E)
         {
             args.SuppressDefaultAction = true;
             args.OriginalArgs.Handled = true;
             this.radRichTextBox.Insert("€");
         }
     };