Language list is not correctly ordered when it's localized

Bugs always need tests ;)

Works fine for me, the patch fix the issue, allowing the language list to be correctly ordered (See attached screenshots)

Nop sorry.
I have some changes to suggest/post but I'm on my phone right now.
I'll post that tomorrow.

Let's comment that in a row.
Two times I lose my comment being stupid, that's enough!

First, your change removes the only call to TranslatableMarkup. You should remove the related "use" at the top of the file.

Then, the good practice is to use StringTranslationTrait in a class to have access to $this->t() instead of calling the t() function directly.
You can have a look at \Drupal\taxonomy\TaxonomyPermissions to have a short example.

See the related change record: https://www.drupal.org/node/2079611

Excellent, that was exactly what I meant.
Thank you, you made a good job :)

By the way, if someone wants to make before/after screenshots, that'd fully complete the issue.
See https://www.drupal.org/contributor-tasks/add-screenshots

True ;)
Can you embed them in the issue summary or in a comment to make them more noticeable?
See steps 7 and 8 of the contributor task I linked in #15

Thanks for the patch, test coverage, and manual testing. Seems like a helpful fix.

+++ b/core/modules/language/src/Tests/LanguageLocaleListTest.php
@@ -0,0 +1,83 @@
+    // Delete French.
+    $this->drupalPostForm('admin/config/regional/language/delete/fr', array(), t('Delete'));

It's not immediately clear to me why this is included at the end of the test.

+++ b/core/modules/language/src/ConfigurableLanguageManager.php
@@ -13,7 +13,7 @@
-use Drupal\Core\StringTranslation\TranslatableMarkup;
+use Drupal\Core\StringTranslation\StringTranslationTrait;

@@ -24,6 +24,8 @@
+  use StringTranslationTrait;

@@ -470,9 +472,9 @@ public function getStandardLanguageListWithoutConfigured() {
-      $predefined[$key] = new TranslatableMarkup($value[0]);
+      $predefined[$key] = $this->t($value[0]);

Why is this change necessary? $this->t() just returns a TranslatableMarkup object so I can't see what the difference is.

#20: I don't think it's needed. I removed it.
#21: you're right, I think we thought that $this->t() returned a string as D7 did but as the TranslatableMarkup objects have a __toString() magic method, the natcasesort() function just works.

Run testbot, run!

I forgot to remove the use statement for StringTranslationTrait.

Thanks for the test-only patch to expose the failure. I only found a few minor points:

1. +++ b/core/modules/language/src/Tests/LanguageLocaleListTest.php
   @@ -0,0 +1,80 @@
   + * Adds a new language with translations and tests language list is correctly
   + * ordered.
   

   Nit: This should be a single line of 80 characters or fewer. Reference: https://www.drupal.org/node/1354#drupal

2. +++ b/core/modules/language/src/Tests/LanguageLocaleListTest.php
   @@ -0,0 +1,80 @@
   +   * Functional tests for adding, editing and deleting languages.
   

   Nit: This should start with a verb. I'd suggest "Tests adding, editing, and deleting languages." (Also note the two commas.) Reference: https://drupal.org/node/1354#functions

3. +++ b/core/modules/language/src/Tests/LanguageLocaleListTest.php
   @@ -0,0 +1,80 @@
   +    $this->assertText('French', 'Language added successfully.');
   

   Two things here:

   1. The assertion message parameter is not necessary and actually obscures details of the assertion.
   2. The assertion seems too broad; generically matching the word "French" could result in false positives.

4. +++ b/core/modules/language/src/Tests/LanguageLocaleListTest.php
   @@ -0,0 +1,80 @@
   +    // Translate Spanish language to french (Espagnol).
   

   Very minor nit: "French" should be capitalized.

NW for the third point in particular.

All fixed :)

Committed/pushed to 8.2.x and cherry-picked to 8.1.x and 8.0.x. Thanks!