Updated Drupal to 8.6. This goes with the following updates because it's possible...

[yaffs-website] / web / core / lib / Drupal / Component / Transliteration / data / de.php

<?php

/**
 * @file
 * German transliteration data for the PhpTransliteration class.
 */

$overrides['de'] = [
  0xC4 => 'Ae',
  0xD6 => 'Oe',
  0xDC => 'Ue',
  0xE4 => 'ae',
  0xF6 => 'oe',
  0xFC => 'ue',
];