String: ficelle

Rope (advanced string): corde

Thread: fil

So string from object would like ficelle d'objet which is yeah, string from object. But the "of" thing in Ocaml is well documented. I vaguely remember them when I had an Ocaml project in my coursework.

No, “de” in French has the same meaning as “of” and sounds exactly as weird here. That’s clearly not where this comes from. Also the translation of string with this meaning is “chaîne de caractères”.

Is «chaîne de caractères» what French programmers call strings? (That would surprise me much as English-speaking programmers calling them ‘strings of characters’ would)

Yeah it's the French word. You can always say just "chaîne" when it's unambiguous. Or you can just use the English word