> I also remember finding it quite strange to read types annotated with a space before the colon (e.g. `let x : int in`); a friend told me that this was due to OCaml's authors being French, which unlike English puts spaces in front of colons in prose.

Maybe, but I find it more likely that they just copied math notation. In mathematics (broadly, not just theoretical CS, also analysis), the type of something is indicated with an infix `:`, like `f : A -> B` is a function from A to B. This colon conventionally has equal spacing on both sides, like a regular infix operator.