Actions on FormDialogs are currently not decorated for i18n.
The reason is in these two methods:

• info.magnolia.ui.dialog.formdialog.FormDialogPresenterImpl#start(final Item item, FormDialogDefinition dialogDefinition, final UiContext uiContext, EditorCallback callback)
  the definition is set and decorated in the super method and is accessible using the getter, but instead of getting the decorated definition, we set the undecorated object to the action executor.

• info.magnolia.ui.dialog.formdialog.FormDialogPresenterImpl#buildView
  instead of passing the decorated definition, we re-decorate the passed parameter.