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 their IP belongs to. If no particular localization dependencies are defined for the customer's location, the store content will display the default currency and language.
The currency used for order processing will depend on the payment gateway configured in the store. Some payments allow order 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.