Localization and string metadata for descriptions

[aphillips]

2.2 Describing what is being paid for https://www.w3.org/TR/2020/CR-payment-request-20201203/#describing-what-is-being-paid-for

Example 2 contains this:

const details = {
  id: "super-store-order-123-12312",
  displayItems: [
    {
      label: "Sub-total",
      amount: { currency: "USD", value: "55.00" },
    },
    {
      label: "Sales Tax",
      amount: { currency: "USD", value: "5.00" },
      type: "tax"
    },
  ],
  total: {
    label: "Total due",
    // The total is USD$65.00 here because we need to
    // add shipping (below). The selected shipping
    // costs USD$5.00.
    amount: { currency: "USD", value: "65.00" },
  },
};

The value label contains natural language text with no language metadata or base direction information. The same usage is found in other example sections that follow. I18N recommends that there be a means of providing this metadata so that the document's processor can display the value correctly.

This comment is probably a duplicate of #327. It is also related to #946