Issue #1826252 by Jeroen, benjf, wilco: Domain language negotiation broken when a base url is configured.

core/modules/language/lib/Drupal/language/HttpKernel/PathProcessorLanguage.php

@@ -155,6 +155,9 @@ class PathProcessorLanguage implements InboundPathProcessorInterface, OutboundPa
             $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(), '/');
       }
     }
     return $path;

core/modules/language/lib/Drupal/language/Tests/LanguageUILanguageNegotiationTest.php

@@ -455,7 +455,7 @@ class LanguageUILanguageNegotiationTest extends WebTestBase {
     $this->rebuildContainer();
 
     // 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.

core/modules/language/lib/Drupal/language/Tests/LanguageUrlRewritingTest.php

@@ -134,7 +134,8 @@ class LanguageUrlRewritingTest extends WebTestBase {
       '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.');
 
     // If we set the port explicitly in url(), it should not be overriden.
@@ -144,7 +145,8 @@ class LanguageUrlRewritingTest extends WebTestBase {
       'base_url' => $request->getBaseUrl() . ':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.');
 
   }