Also Weglot (https://www.weglot.com/guides/squarespace-multilingual-site, which is the supported extension for mulit language sites) does not translate the checkout process..
"And, if you have an ecommerce Squarespace site, your checkout cannot be translated which takes away any user-friendly aspects you were hoping to build with a multilingual website – especially from an ecommerce point of view. As well as your checkout, any automatic emails sent from your website will remain in your original site language."