If you wish to make an in-game purchase for real money, you will notice that all prices are in US dollars.
For example:

This is a default feature, and it cannot be changed for now. All the prices in the game at present are specified in USD, regardless of the country you are in.

It is, however, possible to change your country in the payment details and, in some cases, the purchase currency will adapt.

How to change your country and currency

1. Click on the green button with the price of the item you wish to purchase and then on Show more methods.

2. You will see the country field in the next window.

3. Click on this field to open the dropdown menu and type in your preferred country.

4. You will see that the payment options have changed accordingly.
There are two major fields there: the upper one (Credit cards/Debit cards) and the lower one (other various payment options).

5. If you choose the upper field (i.e pay with a bank card) and click on it, the price will stay in the USD, regardless of the country you have selected.

6. If, however, you choose one of the payment options in the lower field, you will see the price in the currency of your selected country.