Thank you for reaching out to the Community! I'm happy to provide some assistance with this request, to ensure it gets resolved.
To share some insight, Shopify has integrated with over 100 different third-party payment providers to allow you to continue accepting credit cards online. Due to this, I encourage you to check out this complete list of all available payment providers within South Korea.
I see that Sezzle Payments is available and believe that they may offer this installment option. I encourage you however, to connect with them directly. They will be able to share more information with you and advise whether this option is supported.