Default language setting form needs validation

First of all needs issue summary update on why is this a bug. No bugs are mentioned in the summary. If this is a bug, then a test is possible to write which reproduces the bug and proves this solves it.

1. +++ b/core/modules/language/src/LanguageListBuilder.php
   @@ -36,13 +37,21 @@ class LanguageListBuilder extends DraggableListBuilder {
   +   * Stores the configuration factory.
   

   The configuration factory. -- is what we use elsewhere for short.

2. +++ b/core/modules/language/src/LanguageListBuilder.php
   @@ -104,6 +116,10 @@ public function buildRow(EntityInterface $entity) {
   +    if ($entity->id() == $this->languageManager->getDefaultLanguage()->getId()) {
   

   Nice simplification.

Ok, makes sense. Needs tests then :) Thanks!

The test would fail if there was a notice, no? At least testbase has some code to augment that? Not sure where that is turned on for tests.

Looks good! Some minor notes:

1. +++ b/core/modules/language/src/LanguageListBuilder.php
   @@ -129,17 +137,25 @@ public function buildForm(array $form, FormStateInterface $form_state) {
   +      $form_state->setErrorByName('site_default_language', $this->t('Selected language no longer exists.'));
   

   Should be "Selected default language no longer exists." IMHO. We never know if someone alters the form to select a language for some other reason, so the error needs to be more specific.

2. +++ b/core/modules/language/src/Tests/LanguageListTest.php
   @@ -179,6 +179,13 @@ function testLanguageList() {
   +    $this->assertRaw('Selected language no longer exists.', 'A validation message is shown when setting an unexisting language as default.');
   

   Should be assertText(t()). You can remove the second string, the feedback should be fine based on the assertion.

Looks good, thanks!

This issue is a normal bug fix, and doesn't include any disruptive changes, so it is allowed per https://www.drupal.org/core/beta-changes. Committed 2af2fb4 and pushed to 8.0.x. Thanks!

Yay, thanks all!