Well, even when trying to market products for multiple countries, you need to select your default currency itself. That's why we've written Merchant center currency in the field. (See img below):
The conversion is handled by Google, a details article about the same is here: https://support.google.com/merchants/answer/7055540?hl=en
Or here's a example by Google about how the converted currency looks like:
So, you can go ahead, choose the Default currency of your Merchant center (make sure it is same as your store's default currency)