How do you determine a programming language to use on new computer program project?

You would determine that by what your program needs of services and the ease of deploying that program, for example, You make a program in C or Python, and this program is a multi platform (works on Mac OS, Windows, Linux) with C you will need to compile every version of that program to work in a specific system, so you have3 programs, one for every system, but with Java for example, you only need one program and that works in every system since java uses a virtual machine, it all depends on your needs and what you are comfortable in working with, some people are more comfortable working with Python than with Ruby.