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
index c1035ec..75819f4 100644
--- 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
@@ -4,10 +4,10 @@
  * @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.
+ * This is a workaround to make sure the 'Last-Modified' HTTP header is not
+ * overridden by FinishResponseSubscriber. 'Last-Modified' is set 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;
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
index d93ac89..b9b4a7e 100644
--- 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
@@ -4,10 +4,10 @@
  * @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.
+ * This is a workaround to make sure the 'Last-Modified' HTTP header is not
+ * overridden by FinishResponseSubscriber. 'Last-Modified' is set 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.
  * It does so with the saveLastModified() method which has a very high priority,
  * so it can save the defined header really early in a class property. Then
  * later on the setLastModified() method can set the header again using the
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 b3bb53b..1faa0aa 100644
--- a/core/modules/locale/tests/modules/locale_test/locale_test.module
+++ b/core/modules/locale/tests/modules/locale_test/locale_test.module
@@ -5,6 +5,9 @@
  * Simulate a custom module with a local po file.
  */
 
+define('LOCALE_TEST_TIMESTAMP_NEW', REQUEST_TIME - 100);
+define('LOCALE_TEST_TIMESTAMP_OLD', REQUEST_TIME - 300);
+
 /**
  * Implements hook_system_info_alter().
  *
@@ -14,7 +17,7 @@
 function locale_test_system_info_alter(&$info, $file, $type) {
   // Only modify the system info if required.
   // By default the locale_test modules are hidden and have a project specified.
-  // To test the module detection proces by locale_project_list() the
+  // To test the module detection process by locale_project_list() the
   // test modules should mimic a custom module. I.e. be non-hidden.
   if (state()->get('locale.test_system_info_alter')) {
     if ($file->name == 'locale_test' || $file->name == 'locale_test_translate') {
@@ -25,7 +28,7 @@ function locale_test_system_info_alter(&$info, $file, $type) {
 }
 
 /**
- * Helper function to copy a translation file to the translations folder.
+ * Copies 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
@@ -39,8 +42,8 @@ function locale_test_system_info_alter(&$info, $file, $type) {
  * @param string $translation_file
  *   Name of the translation file that will be copied.
  * @param int $timestamp
- *   Optional. Timestamp will be set as the creation date for the copy of
- *   the translation file.
+ *   (optional) Timestamp will be set as the creation date for the copy of
+ *   the translation file. By default the timestamp is the current time.
  */
 function locale_test_copy_translation_file($module, $translation_file, $timestamp = NULL) {
   $translation_file_path = drupal_get_path('module', $module) . '/' . $translation_file;
@@ -49,33 +52,28 @@ function locale_test_copy_translation_file($module, $translation_file, $timestam
   file_unmanaged_copy($translation_file_path, $destination, FILE_EXISTS_REPLACE);
 
   if (!empty($timestamp)) {
-    // Set desired timestamp as creation date for the copied translation file.
+    // Set the timestamp of the newly copied file.
     touch($destination, $timestamp);
   }
 }
 
 /**
- * Helper function to serve a translation file as a result of a page callback.
+ * Serves 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 string $translation_file
+ *   Name of the translation file that will be served.
  * @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);
+function locale_test_serve_translation_file($module, $translation_file, $timestamp) {
+  $translation_file_path = drupal_get_path('module', $module) . '/' . $translation_file;
   $headers = array(
     'Content-Type' => 'text/x-po; charset=utf-8',
     'Last-Modified' => gmdate(DATE_RFC1123, $timestamp),
   );
-  return file_transfer($file->uri, $headers);
+  return file_transfer(drupal_realpath($translation_file_path), $headers);
 }
 
 /**
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
index a24ef2d..15e18fa 100644
--- 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
@@ -7,8 +7,10 @@
 
 /**
  * Implements hook_install().
+ *
+ * Copies the translation file provided by the module to the standard
+ * translations folder and sets timestamp of the new copy.
  */
 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);
+  locale_test_copy_translation_file(LOCALE_TEST_CONTRIB_ONE_MODULE_NAME, LOCALE_TEST_CONTRIB_ONE_PO_FILE, LOCALE_TEST_CONTRIB_ONE_PO_TIMESTAMP_LOCAL);
 }
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
index ab99af1..6b2ee3b 100644
--- 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
@@ -6,6 +6,17 @@
  */
 
 /**
+ * Constants to control how the module will provide translation sources. Locale
+ * test mockup modules are almost identical with each other, these constants
+ * mean the differences between them. They also give us an overview what
+ * scenarios can be tested using these mockup modules.
+ */
+define('LOCALE_TEST_CONTRIB_ONE_MODULE_NAME', 'locale_test_contrib_one');
+define('LOCALE_TEST_CONTRIB_ONE_PO_FILE', 'locale_test_contrib_one-8.x-1.1.de.po');
+define('LOCALE_TEST_CONTRIB_ONE_PO_TIMESTAMP_LOCAL', LOCALE_TEST_TIMESTAMP_OLD);
+define('LOCALE_TEST_CONTRIB_ONE_PO_TIMESTAMP_REMOTE', LOCALE_TEST_TIMESTAMP_NEW);
+
+/**
  * Implements hook_locale_translation_projects_alter().
  *
  * The po file supplied with the module is used as mock remote translation file
@@ -18,8 +29,8 @@ function locale_test_contrib_one_locale_translation_projects_alter(&$projects) {
 
   // 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'] = $url . '/%core/%project/%project-%version.%language.po';
+  if (isset($projects[LOCALE_TEST_CONTRIB_ONE_MODULE_NAME])) {
+    $projects[LOCALE_TEST_CONTRIB_ONE_MODULE_NAME]['info']['interface translation server pattern'] = $url . '/%core/%project/%project-%version.%language.po';
   }
 }
 
@@ -30,11 +41,12 @@ function locale_test_contrib_one_locale_translation_projects_alter(&$projects) {
  */
 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',
+  $items['locale-test-remote-translations/8.x/' . LOCALE_TEST_CONTRIB_ONE_PO_FILE] = array(
+    'title' => 'Testing: !po_file',
+    'title arguments' => array('!po_file' => LOCALE_TEST_CONTRIB_ONE_PO_FILE),
     'page callback' => 'locale_test_contrib_one_remote_translation',
     'access callback' => TRUE,
-    'type' => MENU_CALLBACK,
+    'type' => MENU_NORMAL_ITEM,
   );
   return $items;
 }
@@ -45,5 +57,5 @@ function locale_test_contrib_one_menu() {
 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);
+  return locale_test_serve_translation_file(LOCALE_TEST_CONTRIB_ONE_MODULE_NAME, LOCALE_TEST_CONTRIB_ONE_PO_FILE, LOCALE_TEST_CONTRIB_ONE_PO_TIMESTAMP_REMOTE);
 }