query to retrieve all translations and nodes of a language that has been removed from the system?
I ended executing this function to clean up the content:

function delete_non_existing_language_content() {
  // 1. Get enabled languages
  $languages = \Drupal::languageManager()-&gt;getLanguages();
  $lang_codes = array();
  foreach ($languages as $language) {
    $lang_codes[] = $language-&gt;getId();
  } 
  // 2. Get all the nodes that have langcode not in the language list.  
  $result = \Drupal::database()-&gt;select('node_field_data')
    -&gt;fields('node_field_data')
    -&gt;condition('langcode', $lang_codes, 'NOT IN')
    -&gt;execute();

  // @todo think I can use fetchAllKeyed instead?
  $nids = array_keys($result-&gt;fetchAllAssoc('nid'));

  // 3.Delete all of them:
  // @todo Or possibly update them to language neutral code if possible:
  $storage_handler = \Drupal::entityTypeManager()-&gt;getStorage('node');
  $entities = $storage_handler-&gt;loadMultiple($nids);
  $storage_handler-&gt;delete($entities);

}


This worked for my use case, but I don't think it will be performant for large databases.