It is using a lot of external libraries[1], the major one being the spidermonkey javascript engine.

The Python is used for a pile of code generation, although, AIUI, the Servo team are using more and more Rust macros.

[1]: most of these directories contain submodules pointing to Rust wrappers around other libraries: https://github.com/mozilla/servo/tree/master/src/support

[2]: https://github.com/mozilla/servo/tree/master/src/components/...

Which parts of what? Modern game engines? Nearly all of it, minus often gameplay scripting.

In the hypothetical C++/Rust fusion engine? Whatever hasn't been rewritten yet in Rust, and specifically things that interop with other C++ libraries will remain in C++ for longer times.

Edit: Ohh Servo? Similarly, things that haven't been rewritten yet, specifically javascript interpertation/compiliation. AFAIK Servo is focusing on rewriting layout.