2Checkout does support multiple currencies, but its conversion rates are quite poor. Furthermore, it stores currencies in US Dollars. So a merchant based in the UK who chooses to receive payments in GBP will see these converted to USD for storage. If the merchant chooses to receive payment in GBP, the USD amount will then be converted again to GBP for the wire transfer.
To avoid losses in currency conversion, we recommend only using US Dollars for 2checkout - encourage your customers to pay in USD (their credit card company will convert at a better rate than the one 2Checkout would present to them) and have 2Checkout wire the money to you in USD, for your bank to convert to local currency.