I'm not convinced that 5 zloties is correct - it sounds like baby talk and doesn't sound natural at all in English. The plural of Euro is Euro - and should be likewise with Zloty.
I sort of agree or at least I don't know what is absolutely correct because when or whether to use the plural of a word when referring to "collective nouns" in the English language is a matter of endless debate. Ten deer/duck/pound/penny/dollar/zloty/euro/cent versus ten deers/ducks/pounds/pennies/dollars/zloties/euros/cents can all be used correctly (though there are those that will disagree with this) when used in different contexts in different countries.
It is true that euro is the correct plural as dictated by the EU for use in legislative documents but every country also uses terms colloquially and these need not remain static or be the same. In Ireland, since joining the euro-zone, I can pretty much affirm that it would sound odd to hear someone say I want to change my euro into zloty and that I want to change my euros into zloties would be "much easier on my ear" even if others disagree with the grammatical or legislative correctness.
It is very easy to get bogged down with the semantics of grammar and though I find such debates interesting and fun I would be the last to say there is only one way of saying or writing things. In my opinion one of the beauties of the English language is that it has always continued to evolve and that this is likely to continue.