The best way is find out is to try. Italki gives you a choice of various credit card processors. 

In the United States, it is, as far as I know, always true that if a merchant displays the MasterCard symbol, and you have any kind of card that has a MasterCard symbol on it, they will accept your card--regardless of whether it's a credit card, a debit card, a prepaid debit card, or a gift card. This is true in a store or on a website. I don't know if it's true internationally.

If your card has the MasterCard symbol on it, go to the little icon with a tiny picture of yourself at the top right of the screen, click on it, select "buy credits," choose a processor, and see what happens. If that processor won't accept your MasterCard, try a different one.