All amounts entered into a US tax return need to be entered in US dollars.
When converting foreign currency to US Dollars, you will generally use the prevailing exchange rate, i.e., the spot rate, when the taxpayer received the income. If the income was received evenly throughout the year, you can use a yearly average currency exchange rate.
If a taxpayer receives a refund of foreign taxes that they had previously claimed for the foreign tax credit, in their amended return they will use the exchange rate on the day the taxes were paid, rather than the date of the refund.
See below for more information from the IRS about currency exchange rates.