How can the amount upon which a full year has passed and which is due for zakat be determined, given that the money is saved in different currencies? Is debt deducted from the zakat threshold amount? And how can zakat be paid for past years when there is uncertainty about the money reaching the threshold during those years?

You must strive to determine when your wealth reached the nisab. If you are unable to do so, then act upon your best judgment. Then, pay the obligatory zakat when a full lunar year has passed over the wealth while it is at or above the nisab. As for debts, if you own an asset equal to the debt that you do not need, then consider it in exchange for the debt and pay zakat on your wealth. Otherwise, deduct the debt from your wealth.