Why did you deleted your original topic?
i already suggested solution before.
You should to set domain that cover all your subdomains into your cookies.
For details please read about function "setcookie" and it's "domain" parameter
http://php.net/manual/en/function.setcookie.phpYou should to replace "null" in parameters with your main domain in this function call inside your AbanteCart,
for ex. in file core/engine/language.php
setcookie('language',
$lang_code,
time() + 60 * 60 * 24 * 30,
dirname($request->server['PHP_SELF']),
null,
(defined('HTTPS') && HTTPS)
);
and other places.
Then DELETE ALL COOKIES for YOUR DOMAIN and subdomains in browser and check.