Which one of these languages would be most useful?

German. If you plan to spend any amount of time anywhere in Europe, German is one of the most spoken and sought after languages. It's spoken in Switzerland, Austria, Germany, parts of Italy, Lichtenstein, etc.

German!

"Useful" is a pretty broad term. German is spoken by the most people, but that won't help you if you're trying to find a hospital in Seoul. If you're trying to decide which language to learn it might be more useful to know what else you plan of studying, like music, architecture, electronics, etc., or where you would like to visit, or what kind of women/men you're attracted to.

German and Norwegian

Learning all languages is hard; it's a myth that some languages are easier than others. To become really good, every language will take years of hard work. And you will always be seen as a foreigner.

Since all are difficult, I suggest you choose the language you really feel something for. No matter what it is, you'll enjoy it more if you really care about it, and if you really care about it, you will find a way to use it.

Let me be clear, because I want you to understand this. It is true that if you study Spanish, getting started is very smooth and easy. Starting with a language such as Hebrew or Greek or Korean is harder. But these are utterly trivial and unimportant facts. In all of these cases, getting to an expert level is going to be hard and time-consuming. While one language is harder to get started in than another, all languages eventually plateau. The similarities that seemed to help you at first soon become obstacles to moving further with some languages. That is why I say all languages are essentially of equal difficulty. Folks who never go beyond four or five years in a language have barely begun to learn the language, and this is not what I am talking about. If you want to learn a language, you must expect to spend a lot of time in order to get anywhere. If you do not do this, you wind up just wasting your time, which is perhaps worse than not even starting. One does not try to learn a language; one learns the language. period.

You need to take time to learn a language, and I can not sum it up better than one of the leading philosophers of our time.

"Either do, or do not! There is no 'try'!" --Yoda

Everyone in Norway, Sweden, Germany, and Holland speaks English.

In South Korea, many people speak very little English.

If you are outside South Korea, the Korean won't do you much good. Norwegian will help you understand Danish and Swedish, but it won't do much good outside Scandinavia. Speaking Dutch outside Holland won't get you far.

German is more broadly spoken outside its home country than the other languages. If you want to read old scientific papers, German will help most.

But if you want to research the Vikings, learn a Scandinavian language.

You might also consider the ease of learning the language. Learning Korean will be quite challenging. You could learn German, Norwegian, or Swedish more easily, but you will never pronounce Swedish very well.

I'm pretty sure that out of those languages German is the most spoken followed by Korean, then Dutch, then Swedish, then Norwegian.

They can all be as useful as the next or as useless. It really depends what you're planning on using it for. If you mean by useful which one has the greatest amount of speakers, it's German.