However, in an more straightforward case, both the Java and the Scala programming languages compile their code to the same binary .class files, and they can call methods that were written by the other language. It's generally simpler to write your programs using just one language.
Similarly, many programs use pre-compiled libraries of code (e.g. Windows DLL files) that could have been written in a couple different languages. As soon as we start writing graphical applications using Java, then technically your program will involve multiple languages, because deep down under the hood it's calling some Windows or Mac operating system code to create the window that you see, and that code was probably written in C or C++. However, you wouldn't see any of that original human-readable source code in your programs, since you are just using the binary (machine-readable) code that was produced by someone else earlier. But, in a sense, your overall program/product was created using multiple languages.