How is the life of foreigners in Yanbian/Yanji / CN?

We are planning to move to Yanji. We will study at Yanbian University. As Yanbian is known as the third Korea, I would like to know what life in Yanji is like. Is it best to first learn Chinese or Korean to communicate?

Are you already  in Yanji?  If not, you definitely should learn Mandarin over Korean.  The Chinese people here cannot speak any Korean, while all the ethnic Koreans can speak Mandarin.  Speaking Mandarin will help you more than Korean, as a foreigner.