Bueno, pues lo he solucionado así:
index.php
Código PHP:
if($HTTP_COOKIE_VARS["idioma"] == "cat"){
header("Location: http://www.dominio.com/index-cat.php");
exit;
}elseif ($HTTP_COOKIE_VARS["idioma"] == "esp"){
header("Location: http://www.dominio.com/index-esp.php");
exit;
}elseif ($HTTP_COOKIE_VARS["idioma"] == "eng"){
header("Location: http://www.dominio.com/index-eng.php");
exit;
}
index-cat.php
Código PHP:
header("Content-Type: text/html; charset=utf-8");
setcookie("idioma", "cat", time()+604800);
index-esp.php
Código PHP:
header("Content-Type: text/html; charset=utf-8");
setcookie("idioma", "esp", time()+604800);
index-eng.php
Código PHP:
header("Content-Type: text/html; charset=utf-8");
setcookie("idioma", "eng", time()+604800);
Un saludo