Pretty much all the main survivors of the pilot episode’s plane crash had some big secret they were carrying around, and Sun had perhaps the most interesting of them all: she could actually speak English perfectly, but her husband didn’t know. It served as a good example of how casting bilingual actors can lead to exciting storytelling opportunities. When they asked Kim if she was comfortable speaking in Korean for the show, she was initially hesitant: “I was like, ‘sure, but I am gonna eventually speak in English, right?'” They ended up letting her do both, using her ability to speak two languages as a narrative hook for getting viewers interested in her backstory. 

When Jin found out Sun was able to speak English in season 1, he rejected her. But not only does he eventually forgive her, he starts learning English himself. This helps Jin appreciate Sun’s perspective more, which allows him to realize the importance of getting closer to her. His final words to her in the climactic season 6 episode “The Candidate” are, “I love you, Sun” — said in English. The fact that he didn’t speak in their native language understandably rubbed some fans the wrong way, but it tied nicely into Jin’s arc: Korean may be their native language, the big thing that separates them from the other island characters, but it was while Jin was learning English that the two rekindled their love. For him to speak his dying words in English shows how much closer he’s gotten to Sun, and how open-minded he’s grown over time on the island.