/diy/ - Do It Yourself

Since we seem to have good discussions regarding linguistics, a question.

I am interested in eventually learning to be fluent in two far East Asian languages. The four I have narrowed it down to are Japanese, Korean, Chinese (Mandarin) and Vietnamese. On a scale of one to ten for a native English speaker how hard is each one to learn?

Now, I would want to learn each of the four for various reasons. Japanese because the culture is fascinating to me after getting immersed in Judo and jujitsu. Korean because I have done some research on it and it sounds very unique and pleasant, doesn't seem to have the tonal elements that the rest have and the culture is pretty neat as well. Chinese is kinda eh, I just put it in due to current affairs which doesn't really mean anything. Vietnamese is something of a last addition for me. I've heard several native speakers, and it sounds great. Plus Vietnam is a hub of wonderful food, culture and history.

Which would be the best pick to start out with? Will learning one help learning others?