Missing tests. I bet it'll work better now.

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

+<?php
+
+/**
+ * @file
+ * Definition of Drupal\language\Tests\LanguageCustomConfigurationTest.
+ */
+
+namespace Drupal\language\Tests;
+
+use Drupal\simpletest\WebTestBase;
+use Drupal\Core\Language\Language;
+
+/**
+ * Functional tests for language configuration.
+ */
+class LanguageCustomLanguageConfigurationTest extends WebTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  public static $modules = array('language');
+
+  public static function getInfo() {
+    return array(
+      'name' => 'Custom Language configuration',
+      'description' => 'Adds and configures custom languages.',
+      'group' => 'Language',
+    );
+  }
+
+  /**
+   * Functional tests for adding, editing and deleting languages.
+   */
+  public function testLanguageConfiguration() {
+    global $base_url;
+
+    // Create user with permissions to add and remove languages.
+    $admin_user = $this->drupalCreateUser(array('administer languages', 'access administration pages'));
+    $this->drupalLogin($admin_user);
+
+    // Add custom language.
+    $edit = array(
+      'predefined_langcode' => 'custom',
+    );
+    $this->drupalPost('admin/config/regional/language/add', $edit, t('Add custom language'));
+    // Test validation on missing values.
+    $this->assertText(t('!name field is required.', array('!name' => t('Language code'))));
+    $this->assertText(t('!name field is required.', array('!name' => t('Language name'))));
+    $empty_language = new Language();
+    $this->assertFieldChecked('edit-direction-' . $empty_language->direction, 'Consistent usage of language direction.');
+    $this->assertEqual($this->getUrl(), url('admin/config/regional/language/add', array('absolute' => TRUE)), 'Correct page redirection.');
+
+    // Test validation of invalid values.
+    $edit = array(
+      'predefined_langcode' => 'custom',
+      'langcode' => 'white space',
+      'name' => '<strong>evil markup</strong>',
+      'direction' => LANGUAGE_LTR,
+    );
+    $this->drupalPost('admin/config/regional/language/add', $edit, t('Add custom language'));
+    $this->assertRaw(t('%field may only contain characters a-z, underscores, or hyphens.', array('%field' => t('Language code'))));
+    $this->assertRaw(t('%field cannot contain any markup.', array('%field' => t('Language name'))));
+    $this->assertEqual($this->getUrl(), url('admin/config/regional/language/add', array('absolute' => TRUE)), 'Correct page redirection.');
+
+    // Test validation of existing language values.
+    $edit = array(
+      'predefined_langcode' => 'custom',
+      'langcode' => 'de',
+      'name' => 'German',
+      'direction' => LANGUAGE_LTR,
+    );
+
+    // Add the language the first time.
+    $this->drupalPost('admin/config/regional/language/add', $edit, t('Add custom language'));
+    $this->assertRaw(t(
+      'The language %language has been created and can now be used.',
+      array('%language' => $edit['name'])
+    ));
+    $this->assertEqual($this->getUrl(), url('admin/config/regional/language', array('absolute' => TRUE)), 'Correct page redirection.');
+
+    // Add the language a second time and confirm that this is not allowed.
+    $this->drupalPost('admin/config/regional/language/add', $edit, t('Add custom language'));
+    $this->assertRaw(t(
+      'The language %language (%langcode) already exists.',
+      array('%language' => $edit['name'], '%langcode' => $edit['langcode'])
+    ));
+    $this->assertEqual($this->getUrl(), url('admin/config/regional/language/add', array('absolute' => TRUE)), 'Correct page redirection.');
+  }
+}

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

+<?php
+
+/**
+ * @file
+ * Definition of Drupal\locale\TranslationStream.
+ */
+
+namespace Drupal\locale;
+
+use Drupal\Core\StreamWrapper\LocalStream;
+
+/**
+ * Defines a Drupal translations (translations://) stream wrapper class.
+ *
+ * Provides support for storing translation files.
+ */
+class TranslationsStream extends LocalStream {
+
+  /**
+   * Implements Drupal\Core\StreamWrapper\LocalStream::getDirectoryPath()
+   */
+  function getDirectoryPath() {
+    return variable_get('locale_translate_file_directory',
+      conf_path() . '/files/translations');
+  }
+
+  /**
+   * Implements Drupal\Core\StreamWrapper\StreamWrapperInterface::getExternalUrl().
+   * @throws \LogicException PO files URL should not be public.
+   */
+  function getExternalUrl() {
+    throw new \LogicException('PO files URL should not be public.');
+  }
+}