Multilingual URL Alias Fallback

FallbackAliasStorage.php

<?php

namespace Drupal\my_module;

use Drupal\Core\Language\LanguageInterface;
use Drupal\Core\Language\LanguageManagerInterface;
use Drupal\Core\Path\AliasStorageInterface;

class FallbackAliasStorage implements AliasStorageInterface {

  /**
   * The base AliasStorage class that we're decorating.
   *
   * @var \Drupal\Core\Path\AliasStorageInterface
   */
  protected $subject;

  /**
   * The language manager.
   *
   * @var \Drupal\Core\Language\LanguageManagerInterface
   */
  protected $language_manager;

  public function __construct(AliasStorageInterface $subject, LanguageManagerInterface $language_manager) {
     $this->subject = $subject;
     $this->language_manager = $language_manager;
   }

  /**
   * @inheritdoc
   */
  public function save($source, $alias, $langcode = \Drupal\Core\Language\LanguageInterface::LANGCODE_NOT_SPECIFIED, $pid = NULL) {
    return $this->subject->save($source, $alias, $langcode, $pid);
  }

  /**
   * @inheritdoc
   */
  public function load($conditions) {
    return $this->subject->load($conditions);
  }

  /**
   * @inheritdoc
   */
  public function delete($conditions) {
    return $this->subject->delete($conditions);
  }

  /**
   * @inheritdoc
   */
  public function preloadPathAlias($preloaded, $langcode) {
    return $this->subject->preloadPathAlias($preloaded, $langcode);
  }

  /**
   * @inheritdoc
   */
  public function lookupPathAlias($path, $langcode) {
    if ($langcode == LanguageInterface::LANGCODE_NOT_SPECIFIED) {
       return $this->subject->lookupPathAlias($path, $langcode);
    }
    $context = [
      'langcode' => $langcode,
      'operation' => 'path_alias',
    ];
    $fallbacks = $this->language_manager->getFallbackCandidates($context);
    unset($fallbacks[LanguageInterface::LANGCODE_NOT_SPECIFIED]);
    foreach ($fallbacks as $fallback) {
      $alias = $this->subject->lookupPathAlias($path, $fallback);
      if ($alias) {
        return $alias;
      }
    }
    return FALSE;
  }

  /**
   * @inheritdoc
   */
  public function lookupPathSource($path, $langcode) {
    if ($langcode == LanguageInterface::LANGCODE_NOT_SPECIFIED) {
      return $this->subject->lookupPathSource($path, $langcode);
    }
    $context = [
      'langcode' => $langcode,
      'operation' => 'path_alias',
    ];
    $fallbacks = $this->language_manager->getFallbackCandidates($context);
    unset($fallbacks[LanguageInterface::LANGCODE_NOT_SPECIFIED]);
    foreach ($fallbacks as $fallback) {
      $source = $this->subject->lookupPathSource($path, $fallback);
      if ($source) {
        return $source;
      }
    }
    return FALSE;
  }

  /**
   * @inheritdoc
   */
  public function aliasExists($alias, $langcode, $source = NULL) {
    return $this->subject->aliasExists($alias, $langcode, $source);
  }

  /**
   * @inheritdoc
   */
  public function languageAliasExists() {
    return $this->subject->languageAliasExists();
  }

  /**
   * @inheritdoc
   */
  public function getAliasesForAdminListing($header, $keys = NULL) {
    return $this->subject->getAliasesForAdminListing($header, $keys);
  }

  /**
   * @inheritdoc
   */
  public function pathHasMatchingAlias($initial_substring) {
    return $this->subject->pathHasMatchingAlias($initial_substring);
  }
}

my_module.services.yml

services:
  my_module.path.decorating_alias_storage:
    class: Drupal\my_module\FallbackAliasStorage
    decorates: path.alias_storage
    arguments: ['@my_module.path.decorating_alias_storage.inner', '@language_manager']
    public: false