Many Python packages are written in C. So long as there's a clean interface, I don't see it as a problem.

There's no problem here, the issue is that the journal reviewers were complaining the code wasn't written in R, which seems strange when non-vectorizable algorithms essentially force code to be written in another language (the same is true for python/numpy - sometimes you need to drop to cython).

Do you mean C++?

(It's a pet peeve of mine. The language is called "C++". "cpp" is commonly used as the suffix for C++ source file names, but the name "cpp" also refers to the C preprocessor.)

The common R package for integrating C++ is called "Rcpp", so calling it "cpp" isn't that strange in context.