How do I change the currency of the payment?

Prices in the game are displayed in the currency that is set in your account settings: https://taongafarm.com/account/
To change the currency, in the field Payment currency, select the desired currency from the list and save the changes:

When you proceed with a purchase, your bank will convert the price into your national currency at the current exchange rate, and the sum will be displayed before you confirm the payment. The final price may or may not be displayed in your national currency, depending on your bank or payment service provider.