> in what sense is make_string_comparator_for_locale "really" a function which takes a locale and a string and returns a function from string to ordering?

In the sense that "make_string_comparator" is not a useful concept. Being able to make a "string comparator" is inherently a function of being able to compare strings, and carving out a bespoke concept for some variation of this universal idea adds complexity that is neither necessary nor particularly useful. At the extreme, that's how you end up with Enterprise-style OO codebases full of useless nouns like "FooAdapter" and "BarFactory".

The alternative is to have a consistent, systematic way to turn verbs into nouns. In English we have gerunds. I don't have to say "the sport where you ski" and "the activity where you write", I can just say "skiing" and "writing". In functional programming we have lambdas. On top of that, curried functions are just a sort of convenient contraction to make the common case smoother. And hey, maybe the contraction isn't worth the learning curve or usability edge-cases, but the function it's serving is still important!

> Because of point 3, our codebase has a trivial wrapper to put round functions when your function actually returns a function

That seems either completely self-inflicted, or a limitation of whatever language you're using. I've worked on a number of codebases in Haskell, OCaml and a couple of Lisps, and I have never seen or wanted anything remotely like this.