clone shopify checkout locale

When logged in, go to "Settings > Checkout > Checkout Language", click the "View and customize this translation" link.

A page with a url like this opens: "https://yourshop.myshopify.com/admin/settings/locales/[locale_id]".

copy [locale_id].

Go Back and choose "create a new one".

Open the Javascript console, paste this (change to your shop/locale_id first):

$.get('https://[yourshop].myshopify.com/admin/locale_translations.json?locale_id=[locale_id]', function(data) {
  transformed_lang = {};
  $.each(data.locale_translations, function(i, item){
    transformed_lang[item.english] = item.text;
  });

  $('.ssb').each(function(i, item){
    $(item).find('input').val(transformed_lang[$(item).find('label').text()]);
  });
});

Voilà, it's yours to edit. You have to tab through every single one of the fields in order to effectiviely "get" the strings, but that's small work compared to translating all by yourself.