Can I buy K-pop tickets on Korean sites like Melon or Interpark with a foreign credit card?

Only on the global versions. The Korean-language sites (Melon Ticket, Interpark, Yes24, NOL) expect a Korean payment method, so a foreign card gets rejected at checkout. Use the English global sites instead (Interpark Global, Melon Global, NOL World), which take overseas cards. The tradeoff is the global site usually cannot mail tickets to you, so you collect them at the venue on show day.