MDL-69788 admin: validate language menu configuration.

admin/tests/behat/language_settings.feature

@@ -0,0 +1,34 @@
+@core @core_admin
+Feature: Configure language settings for the site
+  In order to configure language settings for the site
+  As an admin
+  I want to set language settings relevant to my site users
+
+  Scenario: Set languages on language menu
+    Given I log in as "admin"
+    And I navigate to "Language > Language settings" in site administration
+    When I set the field "Languages on language menu" to "en"
+    And I press "Save changes"
+    Then I should not see "Invalid language code"
+
+  Scenario: Reset languages on language menu
+    Given I log in as "admin"
+    And I navigate to "Language > Language settings" in site administration
+    When I set the field "Languages on language menu" to ""
+    And I press "Save changes"
+    Then I should not see "Invalid language code"
+
+  Scenario Outline: Set languages on language menu with invalid language
+    Given I log in as "admin"
+    And I navigate to "Language > Language settings" in site administration
+    When I set the field "Languages on language menu" to "<fieldvalue>"
+    And I press "Save changes"
+    Then I should see "Invalid language code: <invalidlang>"
+    Examples:
+      | fieldvalue | invalidlang |
+      | xx         | xx          |
+      | xx\|Bad    | xx          |
+      | en,qq      | qq          |
+      | en,qq\|Bad | qq          |
+      | en$$       | en$$        |
+      | en$$\|Bad  | en$$        |

lang/en/error.php

@@ -339,6 +339,7 @@
 $string['invalidipformat'] = 'Invalid IP address format';
 $string['invaliditemid'] = 'Incorrect item ID';
 $string['invalidkey'] = 'Incorrect key';
+$string['invalidlanguagecode'] = 'Invalid language code: {$a}';
 $string['invalidlegacy'] = 'Incorrect legacy role definition for type: {$a}';
 $string['invalidmd5'] = 'The check variable was wrong - try again';
 $string['invalidmode'] = 'Invalid mode ({$a})';

lib/adminlib.php

@@ -4864,6 +4864,36 @@ public function __construct() {
         parent::__construct('langlist', get_string('langlist', 'admin'), get_string('configlanglist', 'admin'), '', PARAM_NOTAGS);
     }
 
+    /**
+     * Validate that each language identifier exists on the site
+     *
+     * @param string $data
+     * @return bool|string True if validation successful, otherwise error string
+     */
+    public function validate($data) {
+        $parentcheck = parent::validate($data);
+        if ($parentcheck !== true) {
+            return $parentcheck;
+        }
+
+        if ($data === '') {
+            return true;
+        }
+
+        // Normalize language identifiers.
+        $langcodes = array_map('trim', explode(',', $data));
+        foreach ($langcodes as $langcode) {
+            // If the langcode contains optional alias, split it out.
+            [$langcode, ] = preg_split('/\s*\|\s*/', $langcode, 2);
+
+            if (!get_string_manager()->translation_exists($langcode)) {
+                return get_string('invalidlanguagecode', 'error', $langcode);
+            }
+        }
+
+        return true;
+    }
+
     /**
      * Save the new setting
      *