To translate Squarespace commerce features you'll need a third party paid add-on called Weglot. This can translate all the on-page content and is the only product that I'm aware of that can translate the checkout page. This is because Squarespace lock the checkout page to prevent code, but Weglot changes your DNS records to allow them to translate the site content, including the Squarespace checkout.
I've provided links to the Weglot website where you can take a look at the features. They also have an article that explains how to integrate Weglot with Squarespace.
Note however you translate the website content, there are fixed elements in order notification emails that cannot be translated.
