Can I buy Keisei train tickets with international debit card?

According to the Keisei website FAQ:

Can I use a credit card to purchase tickets?

Major credit cards are accepted only for purchase of a Skyliner ticket (special limited express ticket) or a combined base fare and Skyliner ticket. All other tickets must be purchased in cash using Japanese yen.

This means you can use your card only if you take the expensive faster Skyliner. If you take any of the slower options, you'll have to use cash.

As for which cards are accepted, the Keisei website lists the following:

• Visa
• Mastercard
• JCB
• American Express
• Diners Club
• Union Pay (only at the ticket office)
• Discovery (only at the ticket office)

It says credit card, but if your debit card is under any of those brands, it should theoretically also work.

If you have an Apple iPhone 8 or X and/or an Apple Watch 3 or 4, then you can get the mobile Suica application on your phone or watch and load it up with cash from your debit card using the Suica App.

Then you can use your iPhone or Apple Watch as your transit pass for local lines such as the Keisei (non-Skyliner), subways, and JR lines. The Suica App is one of the few ways to reload cash onto a Suica card using a credit card. Suica and Apple don’t charge any fees for doing this but you should check to see if your bank will charge a fee for a foreign transaction. This is a very handy way to navigate Tokyo (or almost all of Japan, since almost all trains and subways in Japan are compatible with Suica).

There's also two Google/Android Suica apps, although I'm not as familiar with them but it looks like they are similar to the Apple iOS version. Unfortunately, JR doesn't have any information in English but there are other websites that describe how to use them.