Haven't looked at the Bazel codebase, but it would strongly surprise me if the language support rules were implemented in Starlark. More likely, I'd suppose them to be almost exclusively written in Java (and highly based on quite Bazel-specific classes), with Starlark only coming into play in the form of bindings for being used by the users for their BUILD/bzl definitions.

Edit/Add: cf the part about language rules in this comment from a person who says they're a former bazel developer: https://news.ycombinator.com/item?id=35477309

Only the C++ and Java rules are native, the rest are Starlark. I don't know Bazel internals so I can't really comment on your point about APIs.

Same language does not necessarily mean Buck2 will provide the same API as Bazel to write language rules.