Change the default ‘Sale’ text to custom

Goes into functions.php:

function twstudio_wc_custom_replace_sale_text( $html ) {
	return str_replace( __( 'Sale!', 'woocommerce' ), __( '%', 'woocommerce' ), $html );
}
add_filter( 'woocommerce_sale_flash', 'twstudio_wc_custom_replace_sale_text' );