Update for (potential) changes in translation_helpers

#307810: Multilingual support for flagging added multilingual support. Part of the solution was a proposed Drupal core change as presented in #318328: Hook to respond to change of source translation and implemented temporarily in the translation_helpers module.

TODO: revisit this when #318328: Hook to respond to change of source translation is committed. If changes were made to the approach, update to reflect the modified approach.