The saved by and published by messages should contain the date or user name.  Not every language has the same structure, so this information may come before or after (or in the middle) of the other text.

To add variable info you can do:

  'videopagetool-header-right-last-saved' => 'Last saved: $1',

and then when you translate this key you do:

  wfMessage( 'videopagetool-header-right-last-saved',  $wg->lang->timeanddate( $lastSavedOn, true ) )->plain()