No. you use your euros for margin, then when the position is closed your profit (loss) will show as a USD balance (or negative balance). Eventually you may want to convert this back to euros depending on your long term view of the future of the usd, but there is no need to do this often, once a month or two would probably suit most people since there is a small bid-ask spread (but only 1pip on Idealpro if you change >25k, perhaps as part of another trade) and fee $2.50(?) to do so.
I get it, thank you peto.
But how about this: Do they let you trade futures with minimum deposit (10000 usd), or is more required? 10k is not enough for all markets, but it should be enough for mini-corn, mini-wheat etc. I heard that somebody was turned down, because he did not have high enough "net worth."