diff --git a/core/modules/config_translation/src/ConfigNamesMapper.php b/core/modules/config_translation/src/ConfigNamesMapper.php
index 5d15dd5..26c90f6 100644
--- a/core/modules/config_translation/src/ConfigNamesMapper.php
+++ b/core/modules/config_translation/src/ConfigNamesMapper.php
@@ -387,11 +387,11 @@ public function getTypeLabel() {
   public function getLangcode() {
     $config_factory = $this->configFactory;
     $langcodes = array_map(function($name) use ($config_factory) {
-      // Default to English if no language code was provided in the file.
-      // Although it is a best practice to include a language code, if the
-      // developer did not think about a multilingual use-case, we fall back
-      // on assuming the file is English.
-      return $config_factory->get($name)->get('langcode') ?: 'en';
+      // Default to site's default language if no language code was provided in
+      // the file. Although it is a best practice to include a language code, if
+      // the developer did not think about a multilingual use-case, we fall back
+      // on assuming the file is in site's default language.
+      return $config_factory->get($name)->get('langcode') ?: $this->languageManager->getDefaultLanguage()->getId();
     }, $this->getConfigNames());
 
     if (count(array_unique($langcodes)) > 1) {
diff --git a/core/modules/config_translation/src/Tests/ConfigTranslationNonEnglishSiteTest.php b/core/modules/config_translation/src/Tests/ConfigTranslationNonEnglishSiteTest.php
new file mode 100644
index 0000000..e676dcc
--- /dev/null
+++ b/core/modules/config_translation/src/Tests/ConfigTranslationNonEnglishSiteTest.php
@@ -0,0 +1,53 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\config_translation\Tests\ConfigTranslationNonEnglishSite.
+ */
+
+namespace Drupal\config_translation\Tests;
+
+use Drupal\language\Entity\ConfigurableLanguage;
+use Drupal\simpletest\WebTestBase;
+
+/**
+ * Tests config_translation when the site has a non-English default language.
+ *
+ * @group config_translation
+ */
+class ConfigTranslationNonEnglishSiteTest extends WebTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $profile = 'standard';
+
+  /**
+   * {@inheritdoc}
+   */
+  public static $modules = ['language'];
+
+  /**
+   * Tests field_ui interface when the site's default language is not English.
+   */
+  public function testFieldUi() {
+    $account = $this->drupalCreateUser(['administer content types', 'administer node fields', 'administer modules']);
+    $this->drupalLogin($account);
+
+    // Set another language as default. Remove English.
+    ConfigurableLanguage::createFromLangcode('ro')->save();
+    $this->config('system.site')->set('default_langcode', 'ro')->save();
+    ConfigurableLanguage::load('en')->delete();
+
+    // Enable 'locale' and 'config_translation' modules.
+    $edit = [
+      'modules[Multilingual][config_translation][enable]' => 1,
+      'modules[Multilingual][locale][enable]' => 1,
+    ];
+    $this->drupalPostForm('admin/modules', $edit, t('Install'));
+
+    $this->drupalGet('admin/structure/types/manage/article/fields');
+    $this->assertText(t('Manage fields'));
+  }
+
+}