"I need to sell something in multiple currencies. Is this something I can do with Memberstack?"
A Memberstack project can only have 1 default currency. You can select whatever currency you want (EUR or USD, for example), but you can only offer one currency at a time. It's not possible for your customers to select a currency during checkout (E.g. USD or EUR or GBP).
There may be an additional fee imposed by Stripe or your customer's credit card when they pay with a different currency. For example, there will likely be a fee for someone in the US who purchases a membership in Mexican pesos.