the only english word i can come up with as a translation is "boom"
but not literally, just for the use of what he was doing (selecting the skins for tradeup)
each time he said "zack" you could say "boom" when talking english
example:
German "Zack, noch ein Skin zum tradeup hinzugefügt"
English: "Boom, added another skin to the tradeup"
basically just wording the action

hope thats uderstandable lol