From frwiki “Questions techniques”.
In ext.templateDataGenerator.ui.js, the documentation link target is set to 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 Help:TemplateData.