Why does this work? How do I write the same for, say, subtract?

Thanks, my bad.

Making them static does make them inaccessible, but do_test() is still accessible by the user and can end up in an infinite mutual recursion.

Should I move do_test() into the separate tests (although that will introduce duplication)?

That makes sense, thanks for reviewing it.

I have updated the translation now, I hope it is better.

I have added a translation for C; please review, thanks!