Depends what interests you, where you want to work, for how long, etc.

You can probably survive with Java or C# in corporate world for the rest of your career. In webdev you can probably stick to one language, but will have jump between frameworks as new ones become more popular and there's no work in old services anymore. Although that's still likely more than one language (php, js, sql for example)

If you're anywhere near systems administration, you'll have to deal with anything that comes by anyway, so shell, perl, python, ruby are going to be your friends.

So the answer is as usual - it depends.

No. There's no benefit that I can see to limiting yourself to a single language if you don't have to.

I some how submitted this without my content on accident.

https://news.ycombinator.com/item?id=9781555

It is always best to be platform agnostic.

Web dev?

Try learning at various levels: Ruby/Rails, Node/Express, Php/Laravel.