Is knowing Chinese beneficial for a developer/hacker? How?

No more than French or Russian or Swahili. It would only be relevant if the business case demands Chinese language skill.

I worked in an office, where knowing three languages was a mediocre achievement at best. Everyone spoke 4,5,or even 6 languages. Yet, pretty much everyone was hardly making more than someone flipping burgers+ overtime. Knowing language is nothing, unless you can use it in some profitable way. If you are a attaché to China, of course it's useful, but if you slicing noodles in Chinese restaurant,well then hardly so.

It's all fun and games until you see two or more people having a hard time understanding each other. I'm in great awe and disbelief when this colleague I used to work with told me that they are also encountering language barriers with each other because there are more languages / dialects other than mandarin and cantonese, haha.