El plugin no se ha actualizado, por lo tanto los archivos de lenguaje tampoco. Son los mismos que usamos aquí en SoporteMyBB por ejemplo. Es este código:
<?php
/**
 * This file is part of Google SEO plugin for MyBB.
 * Copyright (C) 2008, 2009 Andreas Klauer <Andreas.Klauer@metamorpher.de>
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 *
 */
$l['googleseo_404_widget'] = "<script type=\"text/javascript\">
  var GOOG_FIXURL_LANG = 'es';
  var GOOG_FIXURL_SITE = '{1}';
</script>
<script type=\"text/javascript\"
    src=\"http://linkhelp.clients.google.com/tbproxy/lh/wm/fixurl.js\"></script>";
$l['googleseo_404_notfound'] = "Error 404, archivo no encontrado.";
$l['googleseo_404_wol'] = "Viendo una <a href=\"{1}\">página de error</a>";
$l['googleseo_sitemap_emptyorinvalid'] = "Mapa del sitio vacío o página inválida.";
$l['googleseo_sitemap_pageinvalid'] = "Página del mapa del sitio inválida.";
$l['googleseo_sitemap_disabledorinvalid'] = "Mapa del sitio deshabilitado o inválido.";
$l['googleseo_sitemap_wol'] = "Explorando el <a href=\"{1}\">mapa del sitio XML</a>";
?>