Competitive programming: C++
Interviews: Anything you want unless you are told otherwise.
And most of the companies do not appreciate python to be used while explaining stuff to them
I dont know what 'most companies' means in this context but if you are applying to the more well known ones like Google, Facebook, Amazon etc then python is a perfectly good option.
When it comes to programming languages for interviews the only suggestion I have is to use a more well known language over an obscure one so it becomes easier for the interviewer to understand your thought process. Languages like python, java, c++ etc are great for interviews.