diff --git a/core/modules/config/lib/Drupal/config/Tests/ConfigLocaleOverrideWebTest.php b/core/modules/config/lib/Drupal/config/Tests/ConfigLocaleOverrideWebTest.php
index 9f9579d..0833653 100644
--- a/core/modules/config/lib/Drupal/config/Tests/ConfigLocaleOverrideWebTest.php
+++ b/core/modules/config/lib/Drupal/config/Tests/ConfigLocaleOverrideWebTest.php
@@ -35,14 +35,22 @@ function testSiteNameTranslation() {
     $adminUser = $this->drupalCreateUser(array('administer site configuration', 'administer languages'));
     $this->drupalLogin($adminUser);
 
-    // Add French and make it the site default language.
-    $this->drupalPost('admin/config/regional/language/add', array('predefined_langcode' => 'fr'), t('Add language'));
+    // Add a custom lanugage.
+    $langcode = 'xx';
+    $name = $this->randomName(16);
+    $edit = array(
+      'predefined_langcode' => 'custom',
+      'langcode' => $langcode,
+      'name' => $name,
+      'direction' => '0',
+    );
+    $this->drupalPost('admin/config/regional/language/add', $edit, t('Add custom language'));
 
     $this->drupalLogout();
 
     // The home page in English should not have the override.
     $this->drupalGet('');
-    $this->assertNoText('French site name');
+    $this->assertNoText('XX site name');
 
     // During path resolution the system.site configuration object is used to
     // determine the front page. This occurs before language negotiation causing
@@ -53,8 +61,8 @@ function testSiteNameTranslation() {
     // language negotiation. Ensure that it applies when we access the French
     // front page.
     // @see \Drupal\Core\PathProcessor::processInbound()
-    $this->drupalGet('fr');
-    $this->assertText('French site name');
+    $this->drupalGet('xx');
+    $this->assertText('XX site name');
   }
 
 }
diff --git a/core/modules/config/tests/config_test/config/locale.config.fr.system.site.yml b/core/modules/config/tests/config_test/config/locale.config.fr.system.site.yml
deleted file mode 100644
index 0e4081b..0000000
--- a/core/modules/config/tests/config_test/config/locale.config.fr.system.site.yml
+++ /dev/null
@@ -1 +0,0 @@
-name: 'French site name'
diff --git a/core/modules/config/tests/config_test/config/locale.config.xx.system.site.yml b/core/modules/config/tests/config_test/config/locale.config.xx.system.site.yml
new file mode 100644
index 0000000..4e3fc87
--- /dev/null
+++ b/core/modules/config/tests/config_test/config/locale.config.xx.system.site.yml
@@ -0,0 +1 @@
+name: 'XX site name'