Thanks for posting back to indicate you found the answer. The xamDockManager will in later versions remove a ContentPane if the placeholders are removed but that is done asynchronously. The best way to remove a pane completely from the xamDockManager is as was described in the post that you found where the pane is explicitly closed when its CloseAction is set to RemovePane. Note, in your case since you are readding the panes you might want to set the CloseAction back to Hide assuming you don't want the end user to remove the pane from the control when it is closed.

  List<ContentPane> contentPanes = new List<ContentPane>();

            foreach (object item in this.languagesTabGroupPane.Items)

            {

                ContentPane contentPane = (item.GetType().GetProperty("Pane", BindingFlags.Instance | BindingFlags.NonPublic).GetValue(item, null)) as ContentPane;

                contentPanes.Add(contentPane);;

            }

            foreach(ContentPane contentPane in contentPanes)

            {

                contentPane.CloseAction = PaneCloseAction.RemovePane;

                contentPane.ExecuteCommand(ContentPaneCommands.Close);

            }