Woocommerce - modify order number

functions.php

add_filter( 'woocommerce_order_number', 'arglebargle_set_order_prefix' );
function arglebargle_set_order_prefix( $order_id ) {
    switch ( get_locale() ) {
        case 'hr':
            $prefix = 'HR-';
            break;
        default:
            $prefix = 'SI-';
            break;
    }

    return ($prefix . $order_id);    
}