If you have a supplier who delivers goods in different currencies, you can add their currencies to the profile.

After that’s done and the PO is created, you can choose the currency you want and then place an order. 

The order amount in the currency of the supplier will be visible both on the PO page and in the general list of orders.

The exchange rate for the order to the foreign supplier is used to estimate the preliminary cost of the order, but this is not the final rate to be used for payments.