If you are looking for a place which is not Japan, perhaps Okubo district in Shinjuku ward is the place which meets your criteria. Although there are relatively large Chinatowns in Yokohama and Kobe, these Chinatowns are mostly run by second or third generations and visited by Japanese.
On the other hand, Okubo is a district which is not only for Japanese but also for expats living in Tokyo, especially Koreans. Okubo is the biggest Koreatown in Tokyo. Then, not like Chinatowns in Yokohama and Kobe, Okubo Koreatown is developed by Koreans who emigrated to Japan after WWII.
Then, Okubo districts main gateway is JR East's Shin-Okubo station on Yamanote line. Shin-Okubo station is one of the two stations on Yamanote line which only served by JR East. However, there is Okubo station which is on JR Chuo line nearby. Due to not so many offices are located around the station, most of the passengers getting off at Shin-Okubo are heading Koreatown.