Yes, trading in or with foreign currencies comes with a conversion cost. However, how much a given trade in or with a foreign currency will cost depends on whether you have enabled automatic currency conversion (AutoFX).
You can find the exact fees for both types of conversion in our Fee Schedule.
When a currency conversion occurs on your account, you will see the entries FX Credit and FX Withdrawal in your Account Statement. These entries are not fees, but a reflection of how the foreign currency transaction is booked in your base currency.