Test a program in the same or a different programming language?

1. When I started to learn about testing a program, all I learned is to use a library or package in the same language as the tested program. For example,

   • if a tested program is written in Python, its testing program is also written in Python and uses UnitTest class in the python standard library

   • if a tested program is written in C++, its testing program is also written in C++ and uses some test library in C++.

2. I just saw a project where the tested program is written in C++, and the testing program is written in Python. Instead of making the Python testing program to call the C++ tested source code, the Python testing program loads and executes the executable compiled from the C++ tested program.

General in software engineering, for a project, how is it decided to choose the same or a different programming language to test a program?

Thanks.