Design Document: multi-currency support

[frankie567]

Historically, Polar has only supported USD. This needs to change!

Follow progress here: polarsource/handbook#18

[josealvarez97]

@frankie567 I probably should create a separate issue for this, but any chance that internationalization/localization is also on the roadmap?

Honestly, for what is worth, as a polar customer I wold much rather have localization than multi-currency support.

My users can see the conversion rates through their banks, but they want to make sure they understand what they're paying for during checkout.

It would be great if it were as easy as changing the theme via query parameters (i.e., "?theme=dark")