For a new grad, I recon you checkout the LLVM introduction and its pointers by Adrian Sampson for a primer and then look at the LLVM tutorials [0] about building a toy language. This will give you a feel about how 'modern' compiler development ties in with a programming language using the LLVM infrastructure.

If you are feeling for contributing to a real-world language, there are a few such as Go (very advanced), Rust, Swift, etc. For a beginner, my recommendation would be to checkout the Zig Programming Language [2] as a start and then look at the others.

If you can't choose, look at Awesome Compilers: [3]

This is the sort of question that I am very pleased to see on HN and we need more of.

[0] - https://www.cs.cornell.edu/~asampson/blog/llvm.html

[1] - https://llvm.org/docs/tutorial/

[2] - https://ziglang.org

[3] - https://github.com/aalhour/awesome-compilers