For a systems interview, should I select C, C++, Python, or another language?
Lean towards a high-level language like Python, when possible, especially for data structures and algorithm questions. High-level languages allow programmers to write scripts that are (generally) independent of a particular type of computer. They are closer to human language. While C used to be referred to as high-level, many programmers now consider the language to be low-level, as it lacks a large runtime-system, supports mostly scalar operations, and offers direct memory addressing.
If Python is not your strongest language, then select the one that you perform best in. Focus on improving your skills in the language that you use the most. The likelihood that you will get to choose your language is high, but remember that some languages take longer to code in than others.