what's the second language do you recommend to learn?
Something imperative (i.e. same paradigm) but different. Python is dynamically typed with significant whitespace, so something statically types without significant whitespace: e.g. Java or C#.
These would also make a nice stepping stone towards C. The benefit of C is you really know what's going on, but with the disadvantage that you have to control it all. This level of control is not need for most business problems.
it's really necessary learn a second programming language?
Really subjective, but most good developers know many (consider for a web app: Python, Ruby, C#, Java on the server; SQL on the database and JavaScript on the client; and then the mark-up...).
You benefit from being able to see other approaches to problems and thus create better solutions. So once you have covered more imperative languages move into other paradigms like functional.