In my country many companies wants a developer that can code in at least two back end languages, specially if they are hiring a back end developer (PHP + Ruby is very common)
Learning another language will increase your oportunities in the industry. Your tech stack is going to be more wide so you can choose the right tools for the right job.
This movement make sense to me if you want to improve your career and you already have knowledge about general concepts and fundamentals (Which are language agnostic).
Some people are agree in teach/learn two languages at time, and they have good reasons to do it.
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