Issue #2303525 by paulmckibben, tim.plunkett: Provide link tags to alternate languages (hreflang) in HTML head.

core/modules/content_translation/content_translation.module

@@ -718,3 +718,43 @@ function content_translation_preprocess_language_content_settings_table(&$variab
   module_load_include('inc', 'content_translation', 'content_translation.admin');
   _content_translation_preprocess_language_content_settings_table($variables);
 }
+
+/**
+ * Implements hook_page_alter().
+ */
+function content_translation_page_alter(&$page) {
+  $route_match = \Drupal::routeMatch();
+
+  // If the current route has no parameters, return.
+  if (!($route = $route_match->getRouteObject()) || !($parameters = $route->getOption('parameters'))) {
+    return;
+  }
+
+  // Determine if the current route represents an entity.
+  foreach ($parameters as $name => $options) {
+    if (!isset($options['type']) || strpos($options['type'], 'entity:') !== 0) {
+      continue;
+    }
+
+    $entity = $route_match->getParameter($name);
+    if ($entity instanceof ContentEntityInterface) {
+      // Current route represents a content entity. Build hreflang links.
+      foreach ($entity->getTranslationLanguages() as $language) {
+        $url = $entity->urlInfo()
+          ->setOption('language', $language)
+          ->setAbsolute()
+          ->toString();
+        $page['#attached']['drupal_add_html_head_link'][] = array(
+          array(
+            'rel' => 'alternate',
+            'hreflang' => $language->id,
+            'href' => $url,
+          ),
+          TRUE,
+        );
+      }
+    }
+    // Since entity was found, no need to iterate further.
+    return;
+  }
+}

core/modules/node/src/Tests/NodeTranslationUITest.php

@@ -305,6 +305,9 @@ class NodeTranslationUITest extends ContentTranslationUITest {
 
     // Test that the node page displays the correct translations.
     $this->doTestTranslations('node/' . $node->id(), $values);
+
+    // Test that the node page has the correct alternate hreflang links.
+    $this->doTestAlternateHreflangLinks('node/' . $node->id());
   }
 
   /**
@@ -323,6 +326,28 @@ class NodeTranslationUITest extends ContentTranslationUITest {
     }
   }
 
+  /**
+   * Tests that the given path provides the correct alternate hreflang links.
+   *
+   * @param string $path
+   *   The path to be tested.
+   */
+  protected function doTestAlternateHreflangLinks($path) {
+    $languages = $this->container->get('language_manager')->getLanguages();
+    foreach ($this->langcodes as $langcode) {
+      $urls[$langcode] = url($path, array('absolute' => TRUE, 'language' => $languages[$langcode]));
+    }
+    foreach ($this->langcodes as $langcode) {
+      $this->drupalGet($path, array('language' => $languages[$langcode]));
+      foreach ($urls as $alternate_langcode => $url) {
+        // Retrieve desired link elements from the HTML head.
+        $links = $this->xpath('head/link[@rel = "alternate" and @href = :href and @hreflang = :hreflang]',
+          array(':href' => $url, ':hreflang' => $alternate_langcode));
+        $this->assert(isset($links[0]), format_string('The %langcode node translation has the correct alternate hreflang link for %alternate_langcode: %link.', array('%langcode' => $langcode, '%alternate_langcode' => $alternate_langcode, '%link' => $url)));
+      }
+    }
+  }
+
   /**
    * {@inheritdoc}
    */