Someone who knows multiple languages is called polyglot or multilingual (There can be nuances between two words also.). I'm not sure if we can apply these terms to someone who knows multiple programming languages.
Interestingly, polyglot is used in programming jargon too but it means a program/script written in multiple programming languages.
I found an article that discusses the real value of learning more than one programming language and there are the terms multilingual coder and versatile programmer used. These terms are self-explanatory but are they well-established? Also, I wonder if there is any single word used in tech jargon.
Well, you probably know that learning multiple languages is part of the traditional university track—programmers with degrees in computer science will usually have trained in half a dozen languages by graduation. But what's the value of being a multilingual coder in the workplace, where your projects are less theoretical?
"Not only are languages different tools for different jobs, but they are technologies that shape how you think about programming," says Richard Pattis, a senior lecturer of Informatics at UC Irvine who invented the Karel educational programming language in 1981. But this isn't an outright endorsement of learning two-plus languages—it matters which ones you pick. "Learning similar languages might not progress your thinking much," says Pattis.
To expand their minds, Pattis recommends that versatile programmers learn languages from different language paradigms, whether it be object-oriented languages (e.g., C++/Java), functional languages (e.g., ML and Haskell), scripting languages (e.g., Lisp and Python), logic-based languages (e.g., Prolog), or low-level languages (like C, the Java Virtual Machine or a machine language).
Best Answer
As a Software Developer myself, I can say that there is no commonly used term to describe a person that knows more than one programming language. It is so common-place that it would be peculiar to specifically point that out. That is not to say that there is no term with the meaning you seek, but it would be an obscure one, not often used.
Though I would stop short of calling jxh's answer incorrect, I certainly think that using the term software generalist to describe someone who knows a handful of languages would be a bit of a stretch. Perhaps if they knew an unusually large number of languages this term may fit. Software generalist refers more to a person with knowledge of many different problem domains within computer science.