What language should I brush up on for a software engineering project?

I'd hold off on picking a new language until you can get more details. The instructor or an old course syllabus may tell you whether the technology is assigned or whether each group gets to choose. A course taught in a particular language earlier in the program might continue with it, but that's not guaranteed.

Meanwhile, use whichever of C++, Java, or Python you're most comfortable with to build something small and complete. Reading, debugging, testing, and changing a working program will be useful even if the course uses a different language.

For a big group project, I'd put Git near the top of the prep list. Practice making branches, committing changes, merging, and resolving a simple conflict. Knowing how to read someone else's code and explain your own changes will help too.

You don't need to be equally strong in all three languages before the course starts. Keep your strongest one reasonably fresh and follow along with Python as it's taught.

If the project turns out to be a web app, the concepts may matter more than guessing the language now. Get a basic picture of how a client talks to a server, what an API does, and how an application stores and retrieves data from a database.

A tiny app with one endpoint and some stored data would give you a chance to connect those pieces. You can pick a language you already know rather than learning a whole new stack just in case.

Since you've covered language basics, try practicing the parts that come after writing the first version. Add a test, fix a bug without breaking the test, and write a short explanation of how to run the program. Those habits transfer across languages and are handy when several people share a codebase.