Default all entities to English
You probably want to write this as an update hook, something like this should work:

function MY_MODULE_update_N() {
  foreach (\Drupal::entityTypeManager()-&gt;getDefinitions() as $entityType) {
    $entities = \Drupal::entityTypeManager()-&gt;getStorage($entityType)-&gt;loadMultiple();
    foreach ($entities as $entity) {
      if ($entity-&gt;langcode-&gt;value != 'en') {
        $entity-&gt;set('langcode', 'en');
        $entity-&gt;save();
      }
    }
  }
}


This loops through all entity types and loads all entities for the given entity type, then it updates the language if it is not set to english.

You might want to create a batch process if you have many entities to update.

For more info about how to do that, look here.