From [[https://fr.wikipedia.org/wiki/Wikip%C3%A9dia:Questions_techniques/semaine_37_2015#TemplateData_:_it_is_p.C3.A9nible_.21|frwiki “Questions techniques”]].
In [[https://phabricator.wikimedia.org/diffusion/ETDA/browse/master/modules/ext.templateDataGenerator.ui.js;446d759a7f6d78bf6b1caf5d6f6cc6c6cfc0398f$206|ext.templateDataGenerator.ui.js]], the target is set to [[https://www.mediawiki.org/wiki/Extension:TemplateData|Extension:TemplateData]] which contains untranslated data about the extension.
The `templatedata-helplink-target` message is not used anywhere (or really hidden) and its documentation/name suggest that the link was supposed to use it. Using it, the target page would be configurable and default to the translated page [[https://www.mediawiki.org/wiki/Special:MyLanguage/Help:TemplateData|Help:TemplateData]].