diff --git a/core/modules/language/language.negotiation.inc b/core/modules/language/language.negotiation.inc
index 2182f4d..acc4ed4 100644
--- a/core/modules/language/language.negotiation.inc
+++ b/core/modules/language/language.negotiation.inc
@@ -486,6 +486,9 @@ function language_url_rewrite_url(&$path, &$options) {
               $options['base_url'] = str_replace('https://', 'http://', $options['base_url']);
             }
           }
+
+          // Add Drupal's subfolder from the base_path if there is one.
+          $options['base_url'] .= rtrim(base_path(), '/');
         }
         break;
 
diff --git a/core/modules/language/lib/Drupal/language/Tests/LanguageUILanguageNegotiationTest.php b/core/modules/language/lib/Drupal/language/Tests/LanguageUILanguageNegotiationTest.php
index e929171..f71a866 100644
--- a/core/modules/language/lib/Drupal/language/Tests/LanguageUILanguageNegotiationTest.php
+++ b/core/modules/language/lib/Drupal/language/Tests/LanguageUILanguageNegotiationTest.php
@@ -454,7 +454,7 @@ function testLanguageDomain() {
     $this->drupalPost('admin/config/regional/language/detection/url', $edit, t('Save configuration'));
 
     // Build the link we're going to test.
-    $link = 'it.example.com/admin';
+    $link = 'it.example.com' . rtrim(base_path(), '/') . '/admin';
 
     // Test URL in another language: http://it.example.com/admin.
     // Base path gives problems on the testbot, so $correct_link is hard-coded.
diff --git a/core/modules/language/lib/Drupal/language/Tests/LanguageUrlRewritingTest.php b/core/modules/language/lib/Drupal/language/Tests/LanguageUrlRewritingTest.php
index 3effed4..bea5dc1 100644
--- a/core/modules/language/lib/Drupal/language/Tests/LanguageUrlRewritingTest.php
+++ b/core/modules/language/lib/Drupal/language/Tests/LanguageUrlRewritingTest.php
@@ -136,7 +136,7 @@ function testDomainNameNegotiationPort() {
       'language' => $language,
     ));
 
-    $expected = $index_php ? 'http://example.fr:88/index.php/' : 'http://example.fr:88/';
+    $expected = ($index_php ? 'http://example.fr:88/index.php' : 'http://example.fr:88') . rtrim(base_path(), '/') . '/';
 
     $this->assertEqual($url, $expected, 'The right port is used.');
 
@@ -147,7 +147,7 @@ function testDomainNameNegotiationPort() {
       'base_url' => $GLOBALS['base_url'] . ':90',
     ));
 
-    $expected = $index_php ? 'http://example.fr:90/index.php/' : 'http://example.fr:90/';
+    $expected = $index_php ? 'http://example.fr:90/index.php' : 'http://example.fr:90' . rtrim(base_path(), '/') . '/';
 
     $this->assertEqual($url, $expected, 'A given port is not overriden.');