Nix is really cool, and gets even cooler with NixOS.
To answer your questions: On the second run all packages are saved in the local store, so it's super fast. Language specific dependencies are treated like all other Nix packages, so Python dependencies can depend on Node, etc.
It actually creates a directory with hashes of the packages, so if you run another install that requires a package within that was previously downloaded, it just skips the download too.
