diff --git a/core/modules/taxonomy/tests/fixtures/update/drupal-8.taxonomy-index-2889486.php b/core/modules/taxonomy/tests/fixtures/update/drupal-8.taxonomy-index-2889486.php
new file mode 100644
index 0000000000..2409b38447
--- /dev/null
+++ b/core/modules/taxonomy/tests/fixtures/update/drupal-8.taxonomy-index-2889486.php
@@ -0,0 +1,127 @@
+<?php
+
+/**
+ * @file
+ * Contains database additions to drupal-8.bare.standard.php.gz for testing the
+ * upgrade path of https://www.drupal.org/node/2889486.
+ */
+
+use Drupal\Core\Database\Database;
+
+$connection = Database::getConnection();
+
+// Create an es translation for article nid 1 with a field_tag referencing the
+// same tag as in en.
+$connection->insert('node_field_data')
+  ->fields([
+    'nid',
+    'vid',
+    'type',
+    'langcode',
+    'title',
+    'uid',
+    'status',
+    'created',
+    'changed',
+    'promote',
+    'sticky',
+    'revision_translation_affected',
+    'default_langcode',
+    'content_translation_source',
+    'content_translation_outdated',
+  ])
+  ->values([
+    'nid' => 1,
+    'vid' => 2,
+    'type' => 'article',
+    'langcode' => 'es',
+    'title' => 'Test Article - New title',
+    'uid' => 1,
+    'status' => 1,
+    'created' => 1439730300,
+    'changed' => 1439730369,
+    'promote' => 1,
+    'sticky' => 1,
+    'revision_translation_affected' => 1,
+    'default_langcode' => 0,
+    'content_translation_source' => 'en',
+    'content_translation_outdated' => 0,
+  ])
+  ->execute();
+
+$connection->insert('node_field_revision')
+  ->fields([
+    'nid',
+    'vid',
+    'langcode',
+    'title',
+    'uid',
+    'status',
+    'created',
+    'changed',
+    'promote',
+    'sticky',
+    'revision_translation_affected',
+    'default_langcode',
+    'content_translation_source',
+    'content_translation_outdated',
+  ])
+  ->values([
+    'nid' => 1,
+    'vid' => 2,
+    'langcode' => 'es',
+    'title' => 'Test Article - New title',
+    'uid' => 1,
+    'status' => 1,
+    'created' => 1439730300,
+    'changed' => 1439730369,
+    'promote' => 1,
+    'sticky' => 1,
+    'revision_translation_affected' => 1,
+    'default_langcode' => 0,
+    'content_translation_source' => 'en',
+    'content_translation_outdated' => 0,
+  ])
+  ->execute();
+
+$connection->insert('node__field_tags')
+  ->fields([
+    'bundle',
+    'deleted',
+    'entity_id',
+    'revision_id',
+    'langcode',
+    'delta',
+    'field_tags_target_id',
+  ])
+  ->values([
+    'bundle' => 'article',
+    'deleted' => 0,
+    'entity_id' => 1,
+    'revision_id' => 2,
+    'langcode' => 'es',
+    'delta' => 0,
+    'field_tags_target_id' => 5,
+  ])
+  ->execute();
+
+$connection->insert('node_revision__field_tags')
+  ->fields([
+    'bundle',
+    'deleted',
+    'entity_id',
+    'revision_id',
+    'langcode',
+    'delta',
+    'field_tags_target_id',
+  ])
+  ->values([
+    'bundle' => 'article',
+    'deleted' => 0,
+    'entity_id' => 1,
+    'revision_id' => 2,
+    'langcode' => 'es',
+    'delta' => 0,
+    'field_tags_target_id' => 5,
+  ])
+  ->execute();
diff --git a/core/modules/taxonomy/tests/src/Functional/Update/TaxonomyIndexUpdateTest.php b/core/modules/taxonomy/tests/src/Functional/Update/TaxonomyIndexUpdateTest.php
new file mode 100644
index 0000000000..2ae4a847fc
--- /dev/null
+++ b/core/modules/taxonomy/tests/src/Functional/Update/TaxonomyIndexUpdateTest.php
@@ -0,0 +1,66 @@
+<?php
+
+namespace Drupal\Tests\taxonomy\Functional\Update;
+
+use Drupal\FunctionalTests\Update\UpdatePathTestBase;
+
+/**
+ * Ensure that the taxonomy updates are running as expected.
+ *
+ * @group taxonomy
+ * @group Update
+ * @group legacy
+ */
+class TaxonomyIndexUpdateTest extends UpdatePathTestBase {
+
+  /**
+   * The database connection.
+   *
+   * @var \Drupal\Core\Database\Connection
+   */
+  protected $db;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    parent::setUp();
+    $this->db = $this->container->get('database');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setDatabaseDumpFiles() {
+    $this->databaseDumpFiles = [
+      __DIR__ . '/../../../../../system/tests/fixtures/update/drupal-8.filled.standard.php.gz',
+      __DIR__ . '/../../../../../taxonomy/tests/fixtures/update/drupal-8.taxonomy-index-2889486.php',
+    ];
+  }
+
+  /**
+   * Tests taxonomy term parents update.
+   *
+   * @see taxonomy_update_8703()
+   * @see taxonomy_post_update_populate_taxonomy_index_langcode()
+   */
+  public function testTaxonomyIndexLangcodeUpdate() {
+    $this->assertFalse($this->db->schema()->fieldExists('taxonomy_index', 'langcode'), 'The taxonomy_index table does not have a langcode column.');
+    $index_count = $this->db->query('SELECT COUNT(*) FROM {taxonomy_index} WHERE nid = :nid AND tid = :tid', [
+      ':nid' => 1,
+      ':tid' => 5,
+    ])->fetchField();
+    $this->assertEquals(1, $index_count, '1 entry while two translations reference the same tid.');
+
+    // Run updates.
+    $this->runUpdates();
+
+    $this->assertTrue($this->db->schema()->fieldExists('taxonomy_index', 'langcode'), 'The taxonomy_index table has a langcode column.');
+    $index_count = $this->db->query('SELECT COUNT(*) FROM {taxonomy_index} WHERE nid = :nid AND tid = :tid', [
+      ':nid' => 1,
+      ':tid' => 5,
+    ])->fetchField();
+    $this->assertEquals(2, $index_count, '2 entries after the update, one for each translation.');
+  }
+
+}