Ask HN: Is it even sensible to learn all the mainstream languages?

No it is not necessary and it depends on your ambitions, some people like working with the latest and greatest some people like being trendy and yet others have no interest in the whole things. At some point though the landscape of computing fundamentally changes and the languages have to adapt and generally new ones more well suited for that landscape emerge. If you enjoy staying with the state of the art with computing many times using the latest languages is more complementary. For example their are languages that make parallel programming far more tolerable than some of the older but not old languages such as Java or C#. At some point as parallel becomes more and more important they will gain in popularity. That being said, if that does not interest you, you can always choose to sunset with your current languadge of choice. There will be a period where this is not a lucrative proposition, but as it ages and talent in the technology is lost to attrition, it can be as lucrative if not more than staying with the latest and greatest. For example COBOL developers and mainframe developers are now in high demand, it is not unreasonable to command 200k in either of those fields and they will most likely not be overcrowded as people are not flocking in droves to those technologies. But generally you have to chose one of the two paths, either you stick with what you know and sunset or you become the perpetual learner. I chose the latter because I love following along with where we are heading. But there is no shame in taking the other path. I have many friends that did and they are very happy with their choice.

To sort of change the wording of what others have said, don't think of learning new languages. Learn to solve new problems. Sometimes the problem is best described in a language you don't already know. In that case learn it.

In my case, I've solved many interesting problems over the last decade using C and C++ and an odd shell script here and there. It just so happens those are good languages for solving the problems that I need to solve(which happen to occupy the world in between embedded programming and desktop programming).

I'm a decade+ into my software engineering career. I've learned these, in order: Pascal, C, C++, Java, PHP, Perl, Python. Python is my favorite, but I'm glad I got my CS foundation by writing a lot of C++/Java OO code. For me, it's been helpful to learn several languages and then get deeper in each one. The longer you work in any particular one, the more "hidden gems" you discover and the faster/easier you can reach the optimal solution to whatever problem you're facing.

If you really love learning about new things and picking up languages for the challenge (as I do) then you should totally continue soaking up any documentation you can.

However, do not let the latest trend dictate how you use your knowledge. Programming languages are just tools, and sometimes its hard to remember that it's all in how you use your tool that makes it effective.

You should learn Java, Python, Objective C, and maybe Ruby (on Rails) or PHP. Ruby seems to be in decline, and Python is on the rise.