Extract currency code from $orders
The currency data lives in the commerce_order_total field by default. You can extract that information easily using field_get_items(), though an entity_metadata_wrapper() is just more fun:

$order = commerce_order_load($order_id);
$order_wrapper = entity_metadata_wrapper('commerce_order', $order);

$currency_code = $order_wrapper-&gt;commerce_order_total-&gt;currency_code-&gt;value();