The menu in STK uses the navigationTitle property to store the text to show in the menu. I am using localisation in Magnolia and I noticed that this text could not be translated with adding the locale to the end of the property, for example: navigationTitle_de. When attaching a debugger I found that "navigationTitle" is hardcoded into info.magnolia.module.templatingkit.navigation.LinkTitle. See the code snippet below.

public String getNavigationTitle(){
String navigationTitle = node.getNodeData("navigationTitle").getString();
return StringUtils.defaultIfEmpty(StringUtils.defaultIfEmpty(navigationTitle, node.getTitle()), node.getName());
}