Allow locale to be specified by the developer.

[michelle]

It would be helpful for merchants to be able to specify the locale that the user agent displays the payment sheet in.

Especially as we're discussing custom error messages, which are merchant-supplied, it is really awkward if the message the merchant supplies doesn't match the language (and amount / currency display) in the sheet.

An additional data point we have from Stripe Checkout is that localization only helps conversion if the merchant page and the payment modal are displayed in the same locale.