to alter/translate the 'Print Invoice' button string present on the WooCommerce email

function.php

<?php //Do not add this line of code
add_filter("wt_pklist_alter_document_button_label","wt_pklist_alter_document_button_label_email",10,4);

function wt_pklist_alter_document_button_label_email($print_btn_label,$print,$loco,$template_type){
	if( 'invoice' === $template_type ){
		if( 'email' === $loco ){

			$print_btn_label = __("Print your Invoice","print-invoices-packing-slip-labels-for-woocommerce");
		}
	}

	return $print_btn_label;
}