Give advice!?

Hi everybody!
As you know I have a lot of hobbies. But my favourite is learning different languages. Now I can speak four languages: English, Uzbek, Russian, and Tajik. But it's not enough for me, I want to know a lot. So can you help me with choosing another language? I just don't know what to choose. I'm thinking of learning Korean, but my friends say that it isn't useful. So I need some advice. I' m waiting for your messages! Thanks!