I used to eagerly spend time learning every new language I came across - but that was 1988 or so, and the languages were actually different: Lisp, Prolog, Basic, Pascal, C (not that different from Pascal), various assembly languages, SNOBOL, Perl, APL, C++, Python, TCL ; each had different strengths as a language, and "knowing XXX" meant knowing the syntax and some idioms, but mostly grokking the philosophy. I used to write a Tetris game in each language to make sure I actually understand it.

But now? most languages are basically the same Java/C# with minor variations in semantics and syntax - but you don't "know" C#/Java or even Ruby or Python these days unless you also know the library/framework ecosystem. And knowing the ecosystem is not about understanding the philosophy - it's about rote memorization. And that takes time and memory, rather than insight and understanding.

I'm sticking to C for speed, Python for elegance, and K/J/APL for fun. I really like erlang although I haven't had a chance to work with it. While I occasionally still have to write code in other languages (100 lines in random language every month), looking up the syntax/semantic differences and library documentation feels like a Sisyphean chore rather than acquisition of knowledge.