I am proficient in Python. Which programming language should I learn next (in terms of career)?

There are many programming languages you can learn after python but i recommend you to go with Java it is the most popular and also powerful programming language and also Java programs runs faster then python. If you want to go in the field of software development you can go with java.

You can also learn C programming language if you are interested in Unix bases operating systems like linux. C programming language is also helpful in Ethical Hacking.

Some other programming languages which are helpful is prolog which is for AI.

R, pearl and ruby can be a choise.