I'm 100% those background individuals are humans, nothing more. They may be some sort of Space Korean soldiers, which is why one has a mechanical suit and the other are bald. As for why there is a South Korean flag, who knows. The Colonial Marines are American,but I always sort of hoped that each film would take place farther and farther into the future and that connections to Earth would be little to none, but I don't really mind South Korea playing a role. It may be Yutani, but Yutani was always intended to be Japanese, and I'm not aware if it's a Korean last name. But I guess the Yutani could be named after a Korean who is of Japanese ancestry or perhaps Yutani just becomes a Korean last name in the future.