If I want to optimize my time and energy, which programming language do you recommend spending time mastering for the purpose of technical interviews?
But it's almost entirely irrelevant: reasonable companies will either (a) expect you to code in the language they're hiring for or something at least used within the company, or (b) not care (within reasonable bounds, e.g., don't do your interview in Piet).
They all have their strengths, and some are more suitable than others, when trying to convey certain topics. Some might argue that if you're going into an interview with heavy focus on data structures and algorithms, memory intensive topics, parallel computing, etc. then C++ would be a good choice.
Or if it's a job for enterprise style dev., then maybe Java EE or C#/.NET would be good.
With that said, I think it's more important that you pick the language you know best. Interviews (in my experience) rarely revolve around the technicalities of a language, and more around general topics and conceptual things.
The more you know a language, the less you have to "fight" it, and can focus on the problems at hand.
Python is currently the language _I_ enjoy the most, especially for technical interviews. It's (IMO) extremely intuitive and productive, and frankly almost feels like writing pseudo-code, which I value in the interview-setting. I don't need to waste a lot of time on getting utility code to work, and can spend my energy on the real problems.
TL;DR Use the language you know best / most confident with.