If you invest with an account that's denominated in a different currency than the campaign you're investing in (eg., you use a USD account to invest in a Euro-denominated campaign), your funds will be converted at the time of your investment. If you later receive a refund - say you cancel your investment or the campaign fails - your funds will be converted back at the then-current exchange rate.
For example, let's say you use a USD-denominated account to invest €1000 in an EU campaign. Based on the exchange rate at that time, you send $1100 for your investment. A month later, you cancel your investment. Wefunder will send you back €1000, which will convert to USD at the then-current exchange rate. This may result in you being refunded more or less than the $1100 you initially sent.