diff --git a/core/modules/locale/tests/modules/locale_test/lib/Drupal/locale_test/LocaleTestBundle.php b/core/modules/locale/tests/modules/locale_test/lib/Drupal/locale_test/LocaleTestBundle.php
new file mode 100644
index 0000000..2f9db4a
--- /dev/null
+++ b/core/modules/locale/tests/modules/locale_test/lib/Drupal/locale_test/LocaleTestBundle.php
@@ -0,0 +1,34 @@
+<?php
+
+/**
+ * @file
+ * Contains Drupal\locale_test\LocaleTestBundle.
+ *
+ * This is a workaround for not letting FinishResponseSubscriber to neglect and
+ * override the already configured 'Last-Modified' HTTP header which is set and
+ * needed when a remote translation file is served via a menu callback. So a new
+ * event subscriber is registered here to save the value and set it back.
+ */
+
+namespace Drupal\locale_test;
+
+use Symfony\Component\DependencyInjection\ContainerBuilder;
+use Symfony\Component\HttpKernel\Bundle\Bundle;
+use Symfony\Component\DependencyInjection\Reference;
+
+/**
+ * Locale Test dependency injection container.
+ */
+class LocaleTestBundle extends Bundle {
+
+  /**
+   * Overrides Symfony\Component\HttpKernel\Bundle\Bundle::build().
+   */
+  public function build(ContainerBuilder $container) {
+    // Register our response subscriber to handle finished responses.
+    $container->register('finish_response_subscriber', 'Drupal\locale_test\LocaleTestFinishResponseSubscriber')
+      ->addArgument(new Reference('language_manager'))
+      ->setScope('request')
+      ->addTag('event_subscriber');
+  }
+}
diff --git a/core/modules/locale/tests/modules/locale_test/lib/Drupal/locale_test/LocaleTestFinishResponseSubscriber.php b/core/modules/locale/tests/modules/locale_test/lib/Drupal/locale_test/LocaleTestFinishResponseSubscriber.php
new file mode 100644
index 0000000..e62705e
--- /dev/null
+++ b/core/modules/locale/tests/modules/locale_test/lib/Drupal/locale_test/LocaleTestFinishResponseSubscriber.php
@@ -0,0 +1,41 @@
+<?php
+
+/**
+ * @file
+ * Definition of Drupal\locale_test\LocaleTestFinishResponseSubscriber.
+ *
+ * This is a workaround for not letting FinishResponseSubscriber to neglect and
+ * override the already configured 'Last-Modified' HTTP header which is set and
+ * needed when a remote translation file is served via a menu callback. So this
+ * is a new event subscriber which saves the value and sets it back.
+ */
+
+namespace Drupal\locale_test;
+
+use Drupal\Core\EventSubscriber\FinishResponseSubscriber;
+use Drupal\Core\Language\LanguageManager;
+use Symfony\Component\HttpKernel\Event\FilterResponseEvent;
+use Symfony\Component\HttpKernel\KernelEvents;
+use Symfony\Component\HttpKernel\HttpKernelInterface;
+use Symfony\Component\EventDispatcher\EventSubscriberInterface;
+
+/**
+ * Response subscriber to handle finished responses.
+ */
+class LocaleTestFinishResponseSubscriber extends FinishResponseSubscriber {
+
+  /**
+   * Overrides Drupal\Core\EventSubscriber\FinishResponseSubscriber:onRespond().
+   */
+  public function onRespond(FilterResponseEvent $event) {
+    // Save the 'Last-Modified' HTTP header's value.
+    $response = $event->getResponse();
+    $last_modified = $response->headers->get('Last-Modified');
+    // Parent method will change it to REQUEST_TIME.
+    parent::onRespond($event);
+    if (!empty($last_modified)) {
+      // But here it's the desired one again.
+      $response->headers->set('Last-Modified', $last_modified);
+    }
+  }
+}
diff --git a/core/modules/locale/tests/modules/locale_test/locale_test.info b/core/modules/locale/tests/modules/locale_test/locale_test.info
index f17a400..5277751 100644
--- a/core/modules/locale/tests/modules/locale_test/locale_test.info
+++ b/core/modules/locale/tests/modules/locale_test/locale_test.info
@@ -4,6 +4,7 @@ package = Testing
 version = 1.2
 core = 8.x
 hidden = TRUE
+dependencies[] = locale
 
 ; Definitions for interface translations.
 interface translation project = locale_test
diff --git a/core/modules/locale/tests/modules/locale_test/locale_test.module b/core/modules/locale/tests/modules/locale_test/locale_test.module
index 8f87ddc..b3bb53b 100644
--- a/core/modules/locale/tests/modules/locale_test/locale_test.module
+++ b/core/modules/locale/tests/modules/locale_test/locale_test.module
@@ -25,6 +25,60 @@ function locale_test_system_info_alter(&$info, $file, $type) {
 }
 
 /**
+ * Helper function to copy a translation file to the translations folder.
+ *
+ * Mock modules use this method to copy their po files to the translations
+ * folder. They can also specify a timestamp which will be set for the copied
+ * file in order to be able to test different scenarios with these translation
+ * files.
+ *
+ * @param string $module
+ *   Name of the module providing the translation file that will be copied by
+ *   this method. Will be used for constructing the path for this translation
+ *   file.
+ * @param string $translation_file
+ *   Name of the translation file that will be copied.
+ * @param int $timestamp
+ *   Optional. Timestamp will be set as the creation date for the copy of
+ *   the translation file.
+ */
+function locale_test_copy_translation_file($module, $translation_file, $timestamp = NULL) {
+  $translation_file_path = drupal_get_path('module', $module) . '/' . $translation_file;
+  $translate_directory = variable_get('locale_translate_file_directory', conf_path() . '/files/translations');
+  $destination = $translate_directory . '/' . $translation_file;
+  file_unmanaged_copy($translation_file_path, $destination, FILE_EXISTS_REPLACE);
+
+  if (!empty($timestamp)) {
+    // Set desired timestamp as creation date for the copied translation file.
+    touch($destination, $timestamp);
+  }
+}
+
+/**
+ * Helper function to serve a translation file as a result of a page callback.
+ *
+ * @param string $module
+ *   Name of the module providing the translation file that will be served.
+ * @param string $langcode
+ *   Language code of the translation file.
+ * @param int $timestamp
+ *   Timestamp of date that will be set in the 'Last-Modified' HTTP header.
+ */
+function locale_test_serve_translation_file($module, $langcode, $timestamp) {
+  module_load_include('translation.inc', 'locale');
+  module_load_include('compare.inc', 'locale');
+
+  $projects = locale_translation_get_projects(array($module));
+  $source = locale_translation_source_build($projects[$module], $langcode);
+  $file = locale_translation_source_check_file($source);
+  $headers = array(
+    'Content-Type' => 'text/x-po; charset=utf-8',
+    'Last-Modified' => gmdate(DATE_RFC1123, $timestamp),
+  );
+  return file_transfer($file->uri, $headers);
+}
+
+/**
  * Implements hook_locale_translation_projects_alter().
  *
  * The translation status process by default checks the status of the installed
diff --git a/core/modules/locale/tests/modules/locale_test_contrib_one/locale_test_contrib_one-8.x-1.1.de.po b/core/modules/locale/tests/modules/locale_test_contrib_one/locale_test_contrib_one-8.x-1.1.de.po
new file mode 100644
index 0000000..adf6f55
--- /dev/null
+++ b/core/modules/locale/tests/modules/locale_test_contrib_one/locale_test_contrib_one-8.x-1.1.de.po
@@ -0,0 +1,16 @@
+msgid ""
+msgstr ""
+"Project-Id-Version: Drupal 8\\n"
+"MIME-Version: 1.0\\n"
+"Content-Type: text/plain; charset=UTF-8\\n"
+"Content-Transfer-Encoding: 8bit\\n"
+"Plural-Forms: nplurals=2; plural=(n > 1);\\n"
+
+msgid "January"
+msgstr "Januar_1"
+
+msgid "February"
+msgstr "Februar_1"
+
+msgid "March"
+msgstr "Marz_1"
diff --git a/core/modules/locale/tests/modules/locale_test_contrib_one/locale_test_contrib_one.info b/core/modules/locale/tests/modules/locale_test_contrib_one/locale_test_contrib_one.info
new file mode 100644
index 0000000..6db4613
--- /dev/null
+++ b/core/modules/locale/tests/modules/locale_test_contrib_one/locale_test_contrib_one.info
@@ -0,0 +1,8 @@
+name = Locale test contributed module one
+project = locale_test_contrib_one
+description = Translation test module mocking a contributed module with translation files on remote and local file locations.
+package = Testing
+version = 8.x-1.1
+core = 8.x
+;;hidden = TRUE
+dependencies[] = locale_test
diff --git a/core/modules/locale/tests/modules/locale_test_contrib_one/locale_test_contrib_one.install b/core/modules/locale/tests/modules/locale_test_contrib_one/locale_test_contrib_one.install
new file mode 100644
index 0000000..a24ef2d
--- /dev/null
+++ b/core/modules/locale/tests/modules/locale_test_contrib_one/locale_test_contrib_one.install
@@ -0,0 +1,14 @@
+<?php
+
+/**
+ * @file
+ * Sets up local po file.
+ */
+
+/**
+ * Implements hook_install().
+ */
+function locale_test_contrib_one_install() {
+  $timestamp = REQUEST_TIME - 100;
+  locale_test_copy_translation_file('locale_test_contrib_one', 'locale_test_contrib_one-8.x-1.1.de.po', $timestamp);
+}
diff --git a/core/modules/locale/tests/modules/locale_test_contrib_one/locale_test_contrib_one.module b/core/modules/locale/tests/modules/locale_test_contrib_one/locale_test_contrib_one.module
new file mode 100644
index 0000000..ab99af1
--- /dev/null
+++ b/core/modules/locale/tests/modules/locale_test_contrib_one/locale_test_contrib_one.module
@@ -0,0 +1,49 @@
+<?php
+
+/**
+ * @file
+ * Mocks a custom module with local and remote po files.
+ */
+
+/**
+ * Implements hook_locale_translation_projects_alter().
+ *
+ * The po file supplied with the module is used as mock remote translation file
+ * as well as a local po file. Because the interface translation server pattern
+ * contains the host name of this site it is defined here and not in the .info
+ * file.
+ */
+function locale_test_contrib_one_locale_translation_projects_alter(&$projects) {
+  $url = url(NULL, array('absolute' => TRUE)) . 'locale-test-remote-translations';
+
+  // Set the server URL and pattern to where the remote po file for this module
+  // can be found.
+  if (isset($projects['locale_test_contrib_one'])) {
+    $projects['locale_test_contrib_one']['info']['interface translation server pattern'] = $url . '/%core/%project/%project-%version.%language.po';
+  }
+}
+
+/**
+ * Implements hook_menu().
+ *
+ * Provides a callback for the remote translation file.
+ */
+function locale_test_contrib_one_menu() {
+  $items = array();
+  $items['locale-test-remote-translations/8.x/locale_test_contrib_one-8.x-1.1.de.po'] = array(
+    'title' => 'Testing: locale_test_contrib_one-8.x-1.1.de.po',
+    'page callback' => 'locale_test_contrib_one_remote_translation',
+    'access callback' => TRUE,
+    'type' => MENU_CALLBACK,
+  );
+  return $items;
+}
+
+/**
+ * Page callback; serves remote translation file.
+ */
+function locale_test_contrib_one_remote_translation() {
+  // This is essentially the exact same translation file as the one used for
+  // locale po file testing, but we serve its content through this callback.
+  return locale_test_serve_translation_file('locale_test_contrib_one', 'de', REQUEST_TIME - 100);
+}