First (and failing) step

_config/extensions.yml

@@ -10,6 +10,9 @@ SiteConfig:
 LeftAndMain:
   extensions: 
     ['TranslatableCMSMainExtension']
+LeftAndMain_TreeNode:
+  extensions:
+    ['TranslatableCMSTreeNodeExtension']
 HtmlEditorField_Toolbar:
   extensions:
     ['TranslatableEditorToolbarExtension']

code/controller/TranslatableCMSTreeNodeExtension.php

@@ -0,0 +1,30 @@
+<?php
+/**
+ * @package translatable
+ */
+class TranslatableCMSTreeNodeExtension extends Extension {
+
+	function init() {
+		die("Init TranslatableCMSTreeNodeExtension. "); // this never gets called in the current config
+	}
+
+	/**
+	 * Overrides {@link LeftAndMain_TreeNode->forTemplate()} in order to show the original page name on mouseover.
+	 *
+	 * @todo Make it more flexible, because right now it's a simple copy-paste with changes
+	 *
+	 * @return String
+	 */
+	public function forTemplate() {
+		$obj = $this->owner->obj;
+		$original_pagename = ' (original page name: coming soon!)';
+
+		return "<li id=\"record-$obj->ID\" data-id=\"$obj->ID\" data-pagetype=\"$obj->ClassName\" class=\""
+		. $this->getClasses() . "\">" . "<ins class=\"jstree-icon\">&nbsp;</ins>"
+		. "<a href=\"" . $this->getLink() . "\" title=\"("
+		. trim(_t('LeftAndMain.PAGETYPE','Page type'), " :") // account for inconsistencies in translations
+		. ": " . $obj->i18n_singular_name() . ") {$obj->Title}$original_pagename\" ><ins class=\"jstree-icon\">&nbsp;</ins>"
+		. "<span class=\"text\">{$obj->TreeTitle}</span></a>";
+	}
+
+}