Programmatically added EditorToolGroup to the RadEditor's Ribbonbar is not displayed when the control is localized in Lightweight rendering.
The solution is to define the custom tab / tag / tools in the RadEditor.Tools.resx and RadEditor.Tools.de-DE.resx localization files, e.g.
Here is the configuration of RadEditor:
		<telerik:RadEditor RenderMode="Lightweight" ID="reBody" runat="server" 
 ToolbarMode="RibbonBar" ToolsFile="~/toolbar.xml" 
 LocalizationPath="~/App_GlobalResources/" 
			Language="de-DE" >
			<ImageManager ViewPaths="~/" />
		</telerik:RadEditor>	protected void Page_Load(object sender, EventArgs e)
	{
		EditorToolGroup etgBodyToolbar = new EditorToolGroup();
		reBody.Tools.Add(etgBodyToolbar);
		etgBodyToolbar.Tag = "Common";
		etgBodyToolbar.Tab = "Accessories";
		EditorTool etItem = new EditorTool();
		etItem = new EditorTool();
		etItem.Name = "DocumentProperty";
		etItem.ShowIcon = true;
		etItem.Text = "Document Property";
		etgBodyToolbar.Tools.Add(etItem);
		etItem = new EditorTool();
		etItem.Name = "GalleryImage";
		etItem.ShowIcon = true;
		etItem.Text = "Image Gallery";
		etgBodyToolbar.Tools.Add(etItem);
		etItem = new EditorTool();
		etItem.Name = "ImageManager";
		etItem.ShowIcon = true;
		etItem.Text = "Image Manager";
		etgBodyToolbar.Tools.Add(etItem);
	}