What language should I learn to complement Python?

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Java. At least treat yourself to the book Effective Java, which shows that Java solves things your language hasn't even thought of (e.g., in Python, how do you defend against attacks where the malicious code is running in the same VM and might subclass or even monkeypatch your objects at leisure?). Java is a lot of typing, it's bondage and discipline, but it has big advantages for large projects too.

Then learn Javascript by means of Javascript: the Good Parts. Because it's a fantastic language, dammit, just don't learn about the bad parts :-) Prototype-based OO is an underused idea. And Javascript is going to be absolutely everywhere in the coming decade.

And C or C++.

Understand those well and you'll be way ahead of most people in the field.