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);
}