Use AddFlash function

main/admin/sub_language_add.php

@@ -235,9 +235,7 @@ if (isset($_POST['SubmitAddNewLanguage'])) {
                     SubLanguageManager::remove_language_directory($english_name);
                     $msg .= Display::return_message(get_lang('LanguageDirectoryNotWriteableContactAdmin'), 'error');
                 } else {
-                    // Here we build the confirmation message and we send the user to the sub language terms definition page, using a little hack - see #3712
-                    $_SESSION['msg'] = Display::return_message(get_lang('TheNewSubLanguageHasBeenAdded') . $str_info . 'confirm',
-                        false);
+                    Display::addFlash(Display::return_message(get_lang('TheNewSubLanguageHasBeenAdded') . $str_info));
                     unset($interbreadcrumb);
                     $_GET['sub_language_id'] = $_REQUEST['sub_language_id'] = $sl_id;
                     require 'sub_language.php';