Issue #3128389 by clayfreeman, jungle, cliddell, john.oltman, acbramley, larowlan: [backport] LocaleTranslation is not serializable

core/modules/locale/src/LocaleTranslation.php

@@ -4,6 +4,7 @@ namespace Drupal\locale;
 
 use Drupal\Core\Cache\CacheBackendInterface;
 use Drupal\Core\Config\ConfigFactoryInterface;
+use Drupal\Core\DependencyInjection\DependencySerializationTrait;
 use Drupal\Core\DestructableInterface;
 use Drupal\Core\Language\LanguageInterface;
 use Drupal\Core\Language\LanguageManagerInterface;
@@ -19,6 +20,8 @@ use Symfony\Component\HttpFoundation\RequestStack;
  */
 class LocaleTranslation implements TranslatorInterface, DestructableInterface {
 
+  use DependencySerializationTrait;
+
   /**
    * Storage for strings.
    *

core/modules/locale/tests/src/Kernel/LocaleTranslationTest.php

@@ -0,0 +1,34 @@
+<?php
+
+namespace Drupal\Tests\locale\Kernel;
+
+use Drupal\locale\LocaleTranslation;
+use Drupal\KernelTests\KernelTestBase;
+
+/**
+ * @coversDefaultClass \Drupal\locale\LocaleTranslation
+ * @group locale
+ */
+class LocaleTranslationTest extends KernelTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = [
+    'locale',
+  ];
+
+  /**
+   * Tests that \Drupal\locale\LocaleTranslation is serializable.
+   */
+  public function testSerializable() {
+    $translation = $this->container->get('string_translator.locale.lookup');
+    $this->assertInstanceOf(LocaleTranslation::class, $translation);
+
+    // Prove that serialization and deserialization works without errors.
+    $this->assertNotNull($translation);
+    $unserialized = unserialize(serialize($translation));
+    $this->assertInstanceOf(LocaleTranslation::class, $unserialized);
+  }
+
+}