When installed and configured, the Multicurrency add-on in action will look as follows in the storefront.
A customer will see the currency and the language of the country his IP belongs to. If there are no particular currency and translation defined for the customer's location, the store content will display in the default currency and language.
The currency used for order processing will depend on the payment gateway configured in the store. Some payments allow orders processing only in the default store currency. Others allow accepting payments in more than one currency. Please, check with your payment processing company for details.