A while ago, I met a guy from China who told me that he was ethnic Korean. He spoke Mandarin and what I assume is Yanbianese, since Wikipedia says: "the Korean language Yanbianese use is purely in Hangul [Korean script], without any Hanja [Chinese characters]. Like peninsular Korean language, Yanbianese Korean has Western punctuation, and not Chinese." He also spoke English and Japanese, because he was studying in the U.S. while taking a break from his university in Japan. In other words, he spoke four languages, two fluently and two others very well! An enviable situation!
He said he was from Northeast China, and I forgot where he said, but I'm assuming it's Jilin province, where there's the Yanbian Korean Autonomous Prefecture:
In the 19th century, many Korean immigrants migrated en masse from the Korea Peninsula to China. After the foundation of the Republic of China, a second wave arrived. The population increase was caused by a Japanese invasion in that region. The Japanese were trying to use Korean immigration to diffuse the staying power of Chinese in that region. After the end of World War II, many Koreans in China did not go back to Korea, even though their country has been granted independence. Instead, they joined the Chinese Civil War and were mobilized by communist leader Mao Zedong to fight against the Chinese Nationalist army. As a reward for their involvement, the Communist Party gave Koreans their own autonomous prefecture inside China and divided the land of Han Chinese among the cooperating Koreans.