[browser-payment-api] Add section on internationalization (#53)

Migrating from https://github.com/w3c/webpayments/issues/84:

The spec needs to clearly state how it will handle internationalization issues (such as selection order for language via explicit preferences, Accept-Language headers, etc.)

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/browser-payment-api/issues/53