By default, Piwik reports revenues in $ US dollars.
To change the default currency used by Piwik, login as Super User, then click on ‘Settings’, then ‘Websites’. Below the list of websites, you will find a section ‘Default Currency for new websites’. There you can specify the currency to use by default in Piwik for new websites.
To change the currency of a given website, go to ‘Settings’ then click on ‘Websites’ then click ‘Edit’ next to a website. In the column ‘Currency’ you can choose a new currency for this website.