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/...