Afaik you can not have two prices for the same quantity interval ex 0 .. 0, if overlap, and I remember correctly, the lowest price wins.
Currency conversion is made through convertECB.php or fixed conversion rates in your currency table. F.e 4.00 for zloty and 1.00 for euro.
You can assign prices in both euro and zloty but the quantity interval may not overlap.
Please, keep your temper.
Jörgen @ Kreativ Fotografi