Issue #1934964 by alexpott, Gábor Hojtsy, nevergone: Fixed Locale override subscriber should re-init context to clear caches.

core/lib/Drupal/Core/Config/Context/ConfigContext.php

@@ -63,6 +63,8 @@ class ConfigContext implements ContextInterface {
    * Implements \Drupal\Core\Config\Context\ContextInterface::init().
    */
   public function init() {
+    // Reset existing overrides and get a UUID for this context.
+    $this->overrides = array();
     $this->setUuid();
     // Notify event listeners that a configuration context has been created.
     $this->notify('context', NULL);

core/modules/config/lib/Drupal/config/Tests/ConfigLocaleOverrideWebTest.php

@@ -0,0 +1,60 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\config\Tests\ConfigLocaleOverrideWebTest.
+ */
+
+namespace Drupal\config\Tests;
+
+use Drupal\simpletest\WebTestBase;
+
+/**
+ * Tests language overrides in configuration through the request.
+ */
+class ConfigLocaleOverrideWebTest extends WebTestBase {
+
+  public static $modules = array('locale', 'language', 'system', 'config_test');
+
+  public static function getInfo() {
+    return array(
+      'name' => 'Locale overrides through the request',
+      'description' => 'Tests locale overrides applied through the website.',
+      'group' => 'Configuration',
+    );
+  }
+
+  function setUp() {
+    parent::setUp();
+  }
+
+  /**
+   * Tests translating the site name.
+   */
+  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'));
+
+    $this->drupalLogout();
+
+    // The home page in English should not have the override.
+    $this->drupalGet('');
+    $this->assertNoText('French site name');
+
+    // During path resolution the system.site configuration object is used to
+    // determine the front page. This occurs before language negotiation causing
+    // the configuration factory to cache an object without the correct
+    // overrides. The config_test module includes a
+    // locale.config.fr.system.site.yml which overrides the site name to 'French
+    // site name' to test that the configuration factory is re-initialised
+    // 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');
+  }
+
+}

core/modules/config/tests/config_test/config/locale.config.fr.system.site.yml

@@ -0,0 +1 @@
+name: 'French site name'

core/modules/locale/lib/Drupal/locale/LocaleConfigSubscriber.php

@@ -99,9 +99,11 @@ class LocaleConfigSubscriber implements EventSubscriberInterface {
    *   Kernel event to respond to.
    */
   public function onKernelRequestSetDefaultConfigContextLocale(GetResponseEvent $event) {
-    if ($language = $this->languageManager->getLanguage(LANGUAGE_TYPE_INTERFACE)) {
-      $this->defaultConfigContext->set('locale.language', $language);
-    }
+    // Re-initialize the default configuration context to ensure any cached
+    // configuration object are reset and can be translated. This will invoke
+    // the config context event which will retrieve the negotiated language
+    // from the language manager in configContext().
+    $this->defaultConfigContext->init();
   }
 
   /**