Which second programming language should I learn, C++ or Java? I know Python very well

If you are interested in hacking then you can go with C/C++ but if you are targeting software development you should go with java.

As you mentioned you know python very well, it shows that you are interested in hacking or in AI/Machine Learning so you should go with C/C++ since C and C++ are more low level then java so it will help you in AI as well as in hacking.

choose according to your goals .