core/modules/ckeditor5/ckeditor5.post_update.php   | 26 +++++++++++++++++++++-
 .../src/Plugin/CKEditor5Plugin/CodeBlock.php       |  6 +----
 .../CKEditor5UpdateCodeBlockConfigurationTest.php  |  1 +
 3 files changed, 27 insertions(+), 6 deletions(-)

diff --git a/core/modules/ckeditor5/ckeditor5.post_update.php b/core/modules/ckeditor5/ckeditor5.post_update.php
index 2ebd63f5c7..211478c868 100644
--- a/core/modules/ckeditor5/ckeditor5.post_update.php
+++ b/core/modules/ckeditor5/ckeditor5.post_update.php
@@ -102,6 +102,30 @@ function ckeditor5_post_update_code_block(&$sandbox = []) {
     }
     $settings = $editor->getSettings();
 
-    return in_array('codeBlock', $settings['toolbar']['items'], TRUE);
+    if (in_array('codeBlock', $settings['toolbar']['items'], TRUE)) {
+      $settings = $editor->getSettings();
+      // @see \Drupal\ckeditor5\Plugin\CKEditor5Plugin\CodeBlock::defaultConfiguration()
+      $settings['plugins']['ckeditor5_codeBlock'] = [
+        'languages' => [
+          ['label' => 'Plain text', 'language' => 'plaintext'],
+          ['label' => 'C', 'language' => 'c'],
+          ['label' => 'C#', 'language' => 'cs'],
+          ['label' => 'C++', 'language' => 'cpp'],
+          ['label' => 'CSS', 'language' => 'css'],
+          ['label' => 'Diff', 'language' => 'diff'],
+          ['label' => 'HTML', 'language' => 'html'],
+          ['label' => 'Java', 'language' => 'java'],
+          ['label' => 'JavaScript', 'language' => 'javascript'],
+          ['label' => 'PHP', 'language' => 'php'],
+          ['label' => 'Python', 'language' => 'python'],
+          ['label' => 'Ruby', 'language' => 'ruby'],
+          ['label' => 'TypeScript', 'language' => 'typescript'],
+          ['label' => 'XML', 'language' => 'xml'],
+        ],
+      ];
+      $editor->setSettings($settings);
+    }
+
+    return TRUE;
   });
 }
diff --git a/core/modules/ckeditor5/src/Plugin/CKEditor5Plugin/CodeBlock.php b/core/modules/ckeditor5/src/Plugin/CKEditor5Plugin/CodeBlock.php
index e21b3a7ef6..5b745a35c1 100644
--- a/core/modules/ckeditor5/src/Plugin/CKEditor5Plugin/CodeBlock.php
+++ b/core/modules/ckeditor5/src/Plugin/CKEditor5Plugin/CodeBlock.php
@@ -71,12 +71,8 @@ public function validateConfigurationForm(array &$form, FormStateInterface $form
    *   The parsed equivalent: a list of arrays with each containing:
    *   - label: the label after the pipe symbol, with whitespace trimmed
    *   - language: the key for the language
-   *
-   * @internal
-   *   This method is public only to allow the CKEditor 4 to 5 upgrade path to
-   *   reuse this logic. Mark this private in https://www.drupal.org/i/3239012.
    */
-  public static function parseLanguagesFromValue(string $form_value): array {
+  protected static function parseLanguagesFromValue(string $form_value): array {
     $unparseable_lines = [];
 
     $lines = explode("\n", $form_value);
diff --git a/core/modules/ckeditor5/tests/src/Functional/Update/CKEditor5UpdateCodeBlockConfigurationTest.php b/core/modules/ckeditor5/tests/src/Functional/Update/CKEditor5UpdateCodeBlockConfigurationTest.php
index 2d1540d90c..9bf6f33403 100644
--- a/core/modules/ckeditor5/tests/src/Functional/Update/CKEditor5UpdateCodeBlockConfigurationTest.php
+++ b/core/modules/ckeditor5/tests/src/Functional/Update/CKEditor5UpdateCodeBlockConfigurationTest.php
@@ -39,6 +39,7 @@ public function testUpdateCodeBlockConfigurationPostUpdate(): void {
     $editor = Editor::load('full_html');
     $settings = $editor->getSettings();
     $this->assertArrayHasKey('ckeditor5_codeBlock', $settings['plugins']);
+    // @see \Drupal\ckeditor5\Plugin\CKEditor5Plugin\CodeBlock::defaultConfiguration()
     $this->assertSame([
       'languages' => [
         ['label' => 'Plain text', 'language' => 'plaintext'],