Yes, you can make a donation regardless of your local currency. iDonate pages will display either Euro or Sterling depending on the relevant charity, but anyone can donate from anywhere using any currency.
From US Dollars to Japanese Yen, all currencies are acceptable. What really matters is your payment platform, as donations must be made using VISA, Mastercard, Google Pay, Apple Pay, PayPal or Revolut.
So long as you can make a payment using any of the above, your donation will be accepted. You will need to choose an amount in the charity's currency, but the payment provider will automatically apply the current exchange rate to your funds to charge the equivalent amount.
If you are unsure of the exchange rate, be sure to search online to see how much the donation amount will be in your own currency. Bear in mind, the results may not be perfectly accurate as exchange rates change and vary, but it should give you an estimate from which to base your donation choice.