What are the most popular computer programming languages in observational astronomy?
Python is gaining popularity and replacing MATLAB in many fields of science as the tool for fast prototyping and writing research code.
For example, have a look at:

https://www.astropy.org/
http://www.astropython.org/
https://python4astronomers.github.io/
https://www.astroml.org/
https://asteca.github.io/

(I'm not working in astronomy as a researcher but in some other field.)