Northwest Airlines, Inc. has become the first foreign air carrier to accept ticket reservations and purchases through its Japanese Web site, the company announced Friday.The Internet service covers flights by Northwest Airlines and its partner, KLM Royal Dutch Airlines. Customers can book and buy yen-denominated plane tickets using the online system. After completing the purchasing process on the Internet, they will receive the tickets by mail, the carrier said.Donald Foley, vice president of Northwest Airlines, said the new system aims to provide greater convenience for customers by enabling them to make their travel plans at night and on weekends.Although customers need to conduct the transactions in English, the carrier is currently developing a system in Japanese, the carrier said.