Confusingly, SkinTemplate uses wfMessageFallback to allow menu items to be overridden by skin message keys. However this doesn't apply to all menu items.
* When creating the label action-viewsource for Vector, vector-action-viewsource message will be consulted first, then action-viewsource.
To add further confusion, in several instances the fallback is not intuitive. For example, the history link first consults vector-view-history and then history_short (not view-history)
Menu items that support this are:
* edit and create links (view-edit-local, view-create-local, view-edit, view-create
* $skname-view-foreign / "view-foreign"
* Add section link action-addsection
* View source action-viewsource
* view-history / history_short
* action-delete / delete
* action-move / move
* action-undelete / undelete_short
* viewdeleted / viewdeleted_short
I suggest going forward if this behaviour is necessary:
 the key should always be predictable (e.g. vector-view-history should fallback to view-history )
 this should apply to all menu items (for which T272624 is a blocker)