What is important is your interest of learning. If you love language，whatever it is useful for your work. Just do it. However, you haven't passion about it, just stop it. 

Learning is the thing we should continue the whole life.

The language might be useless to your work, but your other abilities may be improved through the language.

Maybe you can get inspirations from it, because I'm learning English. I have that feeling. Any positive choices should be considered sticking to. 

If you really want to learn it. Don't mind if it's useful. 
And form my points of view, a brand new language that differs from your mother language can offer you a chance to change your mind set. 
Like different grammar or  something. 
but the most interesting thing is that you can "pretend B" (it's a idiomatic Chiglish hahaha).

Firstly, answer your question, my pleasure.

In my opinion, English can bring me new mind to view wolrd, arouse me for desiration of learning, and boraden my horizons. Thus, I deem that it is of great necessity for youngsters of china to study english.

As long as you are interested it, why not give it a try? Maybe someday it would be useful in your life. Even though it's not, you achieve something you want to do, it's still a pleasant thing.

No, any language all have their culture, when you learn a new language you will learn a culture from this new language. When you go to other countries, you will find how useful that language you learned.

I am a good example. I use English very often in my previous job as a technical and project supervisor to communicate with headquarter in Italy, since I left the company and work in another cmpany. I use English only fewer chances, but I still  keep learning it, because I like speaking English. Also I think it may be useful  in the near future at work, and evern let it become a personal skill is a plus for working life.

Giving my experience as an example. WhenＩwas a undergraduate, Ｉwas planning to study at Japan, thusＩlearned Japanes and passed the N2. But now Ｉstudy at a English speaking country, and totally forget how to speak Japanes. 

If you do not use it frequently, you will forget it sooner or later.

Interest is the best booster, it may disturb your mind if materializing what you like too much. Trust me, if you like,stop worring, looking over your shoulders, doubting, hoping for some easy way out, no goaning, no confusing. JUST DO IT!

Work not equal to life, and whether it is worth to spend time on learning a new language is depend on your interests.