diff --git a/admin/tool/mobile/classes/external.php b/admin/tool/mobile/classes/external.php
index eb2bf6723cb..7df769bb8ad 100644
--- a/admin/tool/mobile/classes/external.php
+++ b/admin/tool/mobile/classes/external.php
@@ -168,11 +168,12 @@ class external extends external_api {
                     (only if age verification is enabled).', VALUE_OPTIONAL),
                 'supportemail' => new external_value(PARAM_EMAIL, 'Site support contact email
                     (only if age verification is enabled).', VALUE_OPTIONAL),
-                'autolang' => new external_value(PARAM_INT, 'Whether to detect default language from browser setting.'),
-                'lang' => new external_value(PARAM_LANG, 'Default language for the site.'),
-                'langmenu' => new external_value(PARAM_INT, 'Whether the language menu should be displayed.'),
-                'langlist' => new external_value(PARAM_RAW, 'Languages on language menu.'),
-                'locale' => new external_value(PARAM_RAW, 'Sitewide locale.'),
+                'autolang' => new external_value(PARAM_INT, 'Whether to detect default language
+                    from browser setting.', VALUE_OPTIONAL),
+                'lang' => new external_value(PARAM_LANG, 'Default language for the site.', VALUE_OPTIONAL),
+                'langmenu' => new external_value(PARAM_INT, 'Whether the language menu should be displayed.', VALUE_OPTIONAL),
+                'langlist' => new external_value(PARAM_RAW, 'Languages on language menu.', VALUE_OPTIONAL),
+                'locale' => new external_value(PARAM_RAW, 'Sitewide locale.', VALUE_OPTIONAL),
                 'warnings' => new external_warnings(),
             )
         );