I'll go a bit further with that.
If you only learn one language, you will generally tend to think in the mindset of that language, and have difficulty following to other languages.
Some examples are in order from recent posts on the forum. People who only know C++ tend to think of object life cycles in terms of constructor and destructor, and have difficulty in situations where lifetimes are more complex or where destructors may not exist or are not guaranteed to be called during the program's lifetime. People who only know Java tend to rely heavily on exceptions, and pass-by-reference is a foreign concept, both were some interesting forum topics. People who only know C# tend to not realize the costs involved at a lower level, and take features like primitive boxing for granted.
I read a really good example of this some years back. A group was looking for a tools developer, and their required skills included being able to develop in multiple languages in addition to basic shell scripting. As part of their coding sample they asked the applicant to write some code to be run from the command line, accept a bunch of integers in std in, sort them, and write them back out to std out. Most examples were 10-30 lines of C, C++, perl, python, and other languages. Only one person wrote a simple shell script, "/usr/bin/sort -n".
If you are limited to only one language you will think only in terms that fit that language naturally. If you know many languages, you can think in terms of other languages. There have been hundreds of time I have been writing code in language /x/ and thought to myself, if only I had this feature from language /y/. Sometimes it has been faster and more efficient just to link against a utility library that calls code from another language, just like the example above where the person reused existing functionality from somewhere else, because they could.
Don't get trapped in the single language rut. At a minimum, learn C++, Java, C#, JavaScript, Python, and Perl. Each one will teach you different ways to think about problems. Learn more languages as you have the time and inclination, hopefully pick up a functional language or two as they will teach you a great deal if your life has been limited to imperative languages.
Knowing a language is like having a tool available to you. When you only have a hammer your skills are limited. When you have 5 different kinds of hammers, many types of saws, a collection of drill bits, grinding tools, welding tools, etc., you will be amazingly productive compared to the guy whose only knowledge is how to use a claws hammer. Just because many game engines are written in C++ does not mean that is the only language to learn. No matter what primary language you will use in your day job, simply being aware of many languages will improve your skills and abilities no matter what language you are using at the moment.