non-translatable-alias.php

<?php

/**
 * Implements hook_entity_insert().
 * https://www.drupal.org/node/2689459
 * @param EntityInterface $entity
 */
function hook_entity_insert(EntityInterface $entity) {
  if ($entity instanceof FieldableEntityInterface && $entity->getFieldDefinition('path') && !$entity->getFieldDefinition('path')->isTranslatable()) {
    $path = $entity->get('path')->first();
    $source = '/node/' . $entity->id(); //Source in $path will be null until the insert is complete.
    if(isset($path)) {
      $path = $path->getValue();
      if(!empty($path['alias'])) {
        $alias_storage = \Drupal::service('path.alias_storage');
        $language_manager = \Drupal::service('language_manager');
        foreach ($language_manager->getLanguages() as $langcode => $language) {
          if ($path['langcode'] != $langcode) {
            if ($translation_path = $alias_storage->load([
              'source' => $source,
              'langcode' => $langcode
            ])
            ) {
              $alias_storage->save($source, $path['alias'], $langcode, $translation_path['pid']);
            }
            else {
              $alias_storage->save($source, $path['alias'], $langcode);
            }
          }
        }
      }
    }
  }
}

/**
 * Implements hook_entity_update().
 * https://www.drupal.org/node/2689459
 * @param EntityInterface $entity
 */
function hook_entity_update(EntityInterface $entity) {
  if ($entity instanceof FieldableEntityInterface && $entity->getFieldDefinition('path') && !$entity->getFieldDefinition('path')->isTranslatable()) {
    $alias = '';
    $path = $entity->get('path')->first();
    if(isset($path)) {
      $path = $path->getValue();
      if(isset($path) && !empty($path['alias'])) {
        $alias = $path['alias'];
      } else {
        $alias = $entity->toUrl()->toString();
      }
    } else {
      $alias = $entity->toUrl()->toString();
    }
    $source = '/node/' . $entity->id();
    $alias_storage = \Drupal::service('path.alias_storage');
    $language_manager = \Drupal::service('language_manager');
    foreach ($language_manager->getLanguages() as $langcode => $language) {
      if ($translation_path = $alias_storage->load([
        'source' => $source,
        'langcode' => $langcode
      ])
      ) {
        $alias_storage->save($source, $alias, $langcode, $translation_path['pid']);
      }
      else {
        $alias_storage->save($source, $alias, $langcode);
      }
    }
  }
}


/**
 * Implements hook_entity_delete().
 * @param EntityInterface $entity
 */
function hook_entity_delete(EntityInterface $entity) {
  if ($entity instanceof FieldableEntityInterface && $entity->getFieldDefinition('path') && !$entity->getFieldDefinition('path')->isTranslatable()) {
    $alias_storage = \Drupal::service('path.alias_storage');
    $language_manager = \Drupal::service('language_manager');
    $source = '/node/' . $entity->id();
    foreach ($language_manager->getLanguages() as $langcode => $language) {
      if ($translation_path = $alias_storage->load(['source' => $source, 'langcode' => $langcode])) {
        $alias_storage->delete(['source' => $source, 'langcode' => $langcode]);
      }
    }
  }
}