You can change this yourself in the app:
Go to the card you wish to change and tap the 'i' icon next to it, then select Edit in the top right hand corner.
You can then click on the drop down arrow where the currency is listed and choose the correct currency.
Curve currently supports the following currencies:
GBP, EUR, USD, CNY, JPY, HKD, NZD, SEK, ZAR, NOK, BRL, CAD, ILS, CHF, SGD, INR, RUB, AUD, BGN, CZK, DKK, HRK, HUF, ISK, PLN, RON.
If you select a currency which isn't listed here, your card will be charged in GBP.