Infusionsoft is not build for handling multiple currencies. The only currency related feature in Infusionsoft is the currency label to use throughout the app and on your order forms. The only thing that determines what currency your customer are charged in is how your Merchant Account is setup.
I’m not sure how the PayPal integration with Infusionsoft works so far as which currency is used. I imagine that Infusionsoft uses the currency you have selected within Infusionsoft and tells PayPal to charge in that currency, but it might just use the default currency you have set in PayPal.
I am not aware of any way of allowing multiple currencies on an order form. The only way I know of charging in different currencies is to have mutliple merchant accounts, one for each currency, and set up one order form per merchant account and set each order form to use the correct merchant account for the currency being charged.