Add order currency to Fortnox order-data

functions.php

<?php
/**
 * @snippet       WooCommerce - Fortnox plugin by Redlight Media - Add order currency to Fortnox
 * @author        Redlight Media AB / Christopher Hedqvist
 * @compatible    WooCommerce 3.3.5
 */
function redlight_fortnox_order_currency( $orderData, $order_id) {
    $order = new WC_Order($order_id);
    $orderData['Order']['Currency'] = $order->get_currency();
    return $orderData;
}
add_filter('obj_fortnox_order_data', 'redlight_fortnox_order_currency', 100, 2 );