How change date format to spanish?

How can change the format of the date to spanish?

This is display in products:
Agregado el: Wednesday 11 April, 2007

• it must be shown in this way
  Agregado el: **Miercoles 11 de Abril del 2007

**When send the order by email appears:
Fecha del pedido: Saturday 14 April, 2007 * it must be shown in this way
Fecha del pedido: Sabado 14 de Abril del 2007

What files must be change?

Best regards from Monterrey, Mexico!

ok, i have this changes in function_general.php

  function zen_date_long($raw_date) {
    if ( ($raw_date == '0001-01-01 00:00:00') || ($raw_date == '') ) return false;

    $year = (int)substr($raw_date, 0, 4);
    $month = (int)substr($raw_date, 5, 2);
    $day = (int)substr($raw_date, 8, 2);
    $hour = (int)substr($raw_date, 11, 2);
    $minute = (int)substr($raw_date, 14, 2);
    $second = (int)substr($raw_date, 17, 2);
    $lemp = "";
    $l_mes = "";
    switch ($month){
        case "01"; $l_mes = "Enero"; break;
        case "02"; $l_mes = "Febrero"; break;
        case "03"; $l_mes = "Marzo"; break;
        case "04"; $l_mes = "Abril"; break;
        case "05"; $l_mes = "Mayo"; break;
        case "06"; $l_mes = "Junio"; break;
        case "07"; $l_mes = "Julio"; break;
        case "08"; $l_mes = "Agosto"; break;
        case "09"; $l_mes = "Septiembre"; break;
        case "10"; $l_mes = "Octubre"; break;
        case "11"; $l_mes = "Noviembre"; break;
        case "12"; $l_mes = "Diciembre"; break;
    }
    $l_emp = $day." de ".$l_mes." del ".$year;
    return $l_emp;
//    return strftime(DATE_FORMAT_LONG, mktime($hour,$minute,$second,$month,$day,$year));
  }