I have written books using a variety of languages that I also have used in work projects. I enjoy variety.

However, as a practical matter, learning just Java and Python really well will cover most areas of software development.

My personal preferences: I prefer Ruby over Python as a scripting language, but I am forced to use Python in my machine learning job.

I have used Common Lisp since about 1984, love it, and I am fortunate to get to occasionally still use it.

I like Haskell, but I am a ‘forever student’ and use a subset of the language that I understand.

EDIT: some knowledge of JavaScript is also recommended