Merge forwardport of #11493 to 2.3-develop branch

Applied pull request patch https://github.com/magento/magento2/pull/11493.patch (created by @JeroenVanLeusden) based on commit(s):
  1. 221c593

Author: magento-engcom-team

app/code/Magento/Checkout/i18n/en_US.csv

@@ -176,3 +176,4 @@ Payment,Payment
 "Not yet calculated","Not yet calculated"
 "We received your order!","We received your order!"
 "Thank you for your purchase!","Thank you for your purchase!"
+"optional", "optional"