For anyone interested in doing this (and taking into account that I am not by any means a PHP coder), I finally hacked a solution that seems to work without breaking anything.
Editing the index.php to accept a parameter via GET which basically switches between one version of the config.php or another depending on the language parameter being passed.
So replacing the line in the index.php:
require_once __DIR__ . '/config.php';
with the code:
$lang=$_GET['language'];
if($lang && file_exists(__DIR__ . '/config-'.$lang.'.php')) require_once __DIR__ . '/config-'.$lang.'.php';
else require_once __DIR__ . '/config.php';
then duplicating the config.php and renaming it to config-whateverlanguageyouwant.php (in my case config-spanish.php)
and finally changing the line in the new config file from:
const LANGUAGE = 'english';
to reflect whatever the alternative language is. So in my case:
const LANGUAGE = 'spanish';