In your settings (Settings > Accounting), you can define your default currency...

...But you can also define a specific currency for each one of your clients in the Clients tab.

All your documents for this client will be generated in this specific currency. 

In the reports, there is one line for each currency you defined.

You can also filter the reports on one currency in particular and you can tick a box to convert all the amounts in the reports into your default currency. For the amounts already paid, the system will use the exchange rate that was applicable at the payment date. For the amounts that are not paid yet, the system will use the last available exchange rate.