Polylang frontend-nav-menu.php fix

/* throws warning */
    public function nav_menu_locations($menus) {
        if (is_array($menus))
            $theme = get_option('stylesheet');

            foreach ($menus as $loc => $menu) {
                if (!empty($this->options['nav_menus'][$theme][$loc][$this->curlang->slug]))
                    $menus[$loc] = $this->options['nav_menus'][$theme][$loc][$this->curlang->slug];
            }

        return $menus;
    }
/* no warnings */
    public function nav_menu_locations($menus) {
        if (is_array($menus)) {
            $theme = get_option('stylesheet');

            foreach ($menus as $loc => $menu) {
                if (!empty($this->options['nav_menus'][$theme][$loc][$this->curlang->slug]))
                    $menus[$loc] = $this->options['nav_menus'][$theme][$loc][$this->curlang->slug];
            }
        }
        return $menus;
    }