In this section:

How do I refund a customer when he has paid by bank transfer? 

When the payment is made by bank transfer, in order to refund the amount, you must transfer it from your account to the customer's account. It will be noted in the system that this action must be performed when you press the refund button and the status of the order becomes Refunded. 

Can my bank account be in a different currency than the one in which I sell the products in my store? 

Yes, you can specify an account in a different currency, but this way you will have conversion fees, etc. If you sell in euro we advise you your account to be in euro too.