Show button to translate extra fields when a sub language is the platform language - refs BT#11015

main/inc/lib/extra_field.lib.php

@@ -1783,11 +1783,11 @@ EOF;
 
         $form->addElement('header', $header);
 
-        if ($action === 'edit') {
         $platformLanguage = api_get_setting('platformLanguage');
         $languageId = api_get_language_id($platformLanguage);
         $languageInfo = api_get_language_info($languageId);
 
+        if ($action === 'edit' && $languageInfo['parent_id']) {
             $translateUrl = api_get_path(WEB_CODE_PATH) . 'admin/sub_language.php?' . http_build_query([
                 'id' => $languageInfo['parent_id'],
                 'action' => 'registersublanguage',