Issue #2108599 by alexpott, Gábor Hojtsy, vijaycs85: Convert language_default to CMI.

core/core.services.yml

core/includes/install.core.inc

core/includes/install.inc

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

core/lib/Drupal/Core/Config/ConfigFactory.php

core/lib/Drupal/Core/DrupalKernel.php

core/lib/Drupal/Core/Language/LanguageDefault.php

core/lib/Drupal/Core/Language/LanguageManager.php

core/lib/Drupal/Core/Language/LanguageManagerInterface.php

...odules/block/lib/Drupal/block/Tests/BlockLanguageTest.php

...config/lib/Drupal/config/Tests/ConfigLanguageOverride.php

...lib/Drupal/config/Tests/ConfigLanguageOverrideWebTest.php

core/modules/language/language.install

core/modules/language/language.module

core/modules/language/language.services.yml

...guage/lib/Drupal/language/ConfigurableLanguageManager.php

.../Drupal/language/ConfigurableLanguageManagerInterface.php

.../modules/language/lib/Drupal/language/Entity/Language.php

.../lib/Drupal/language/EventSubscriber/ConfigSubscriber.php

...al/language/EventSubscriber/LanguageRequestSubscriber.php