Altering existing route for translation overview page ie: entity.node.content_translation_overview
It was not working because in content_transaltion module priority was set to -210 so make need to change priority of custom module to -220 and it started working.
public static function getSubscribedEvents() {
    $events = parent::getSubscribedEvents();
    $events[RoutingEvents::ALTER] = ['onAlterRoutes', -220];
    return $events;
  }