The spellchecker does not recognize words with non-breaking spaces between them as separate.
A workaround when pasting text into the RadRichTextBox is to subscribe to the CommandExecuting event of the control, get the paste text from the clipboard and replace all non-brekaing spaces with spaces.

private void editor_CommandExecuting(object sender, Telerik.Windows.Documents.RichTextBoxCommands.CommandExecutingEventArgs e)
        {
            if (e.Command is PasteCommand)
            {
                var pasteText = Clipboard.GetDataObject().GetData("HTML Format") as string;
                if (pasteText != null)
                {
                    pasteText=  pasteText.Replace("&nbsp", " ").Replace(' ',' ');
                    var originalDataObject = Clipboard.GetDataObject();
                    DataObject dataObject = new DataObject();
                    foreach (string format in originalDataObject.GetFormats())
                    {
                        dataObject.SetData(format, originalDataObject.GetData(format));
                    }
                    dataObject.SetData("HTML Format", pasteText);
                    Clipboard.SetDataObject(dataObject);
                }               
            }
        }