Add availableLanguages field to nodes

Closes #3432965

modules/graphql_compose_available_translations/src/Plugin/GraphQL/DataProducer/AvailableLanguages.php

+<?php
+
+namespace Drupal\graphql_compose_available_translations\Plugin\GraphQL\DataProducer;
+
+use Drupal\graphql\Plugin\GraphQL\DataProducer\DataProducerPluginBase;
+use Drupal\node\NodeInterface;
+
+/**
+ * Returns the translation strings.
+ *
+ * @DataProducer(
+ *   id = "available_languages",
+ *   name = @Translation("Load string translations"),
+ *   description = @Translation("Loads a list of string translations."),
+ *   produces = @ContextDefinition("any",
+ *     label = @Translation("String translation connection")
+ *   ),
+ *   consumes = {
+ *     "entity" = @ContextDefinition("entity:node",
+ *       label = @Translation("Entity"),
+ *     ),
+ *   }
+ * )
+ */
+class AvailableLanguages extends DataProducerPluginBase {
+
+  /**
+   * Resolves the available languages.
+   *
+   * @param \Drupal\node\NodeInterface $node
+   *   The current node.
+   *
+   * @return array
+   *   An array of available languages.
+   */
+  public function resolve(NodeInterface $node): array {
+    // Get the translation languages.
+    $languages = $node->getTranslationLanguages();
+
+    $translations = [];
+    foreach ($languages as $language) {
+      // Check if node has an ID.
+      if (!$node->id()) {
+        continue;
+      }
+      // Get the translated node for each language.
+      $translatedNode = $node->getTranslation($language->getId());
+      // Customize the logic to fetch available languages for the node.
+      $translations[] = [
+        'url' => (string) $translatedNode->toUrl()->toString(TRUE)->getGeneratedUrl(),
+        'language' => (string) $language->getId(),
+      ];
+    }
+
+    return $translations;
+  }
+
+}