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One candidate is ATS [1].

[1]: https://en.wikipedia.org/wiki/ATS_(programming_language)

There's a formally verified C compiler, IIRC the frontend isn't, but if you define the language to the structs that are in the inputs to whatever is formally verified I guess whatever C like dialect of a language it implements must be.

For the seL4 proofs a subset of C was formalized, for example.

(Already mentioned) CakeML would be another example, together maybe with its Pancake sibling.

Also: WebAssembly!