You are able to run your referral campaign in a default language other than English. Please note that changing your default language will reset the templates of your emails & pages.
The latest list of available languages can be found here.
If you are on the updated dashboard (how can I tell?):
Go to the Store Settings section of your dashboard
Under Language Settings, select the default language that you would like to run your referral campaign in
Click the Change default language button to save your changes
If you are on the previous dashboard (how can I tell?):
Go to the Email & Pages section of your dashboard
Go to the 'Language' row in the Campaign Settings box on the right and select the default language that you'd like your referral program to be in
Click the blue "Save" button to save your changes you are done to save your changes.