In planning options -> preferences -> misc.-> fuel prices, I can't find where to set the currency. I'd like to see fuel prices in EUR, but somehow my flight costs show up in USD. Help please!

It's simple a currency symbol issue, nothing else. No currency conversions are done by SkyDemon. We just take the actual value, which could be 1.56 for example, and hand it over to the operating system for display in local currency.