How long ago was this "relatively recently"? When I translated all the remaining French comments in the OCaml compiler nearly 10 years ago, there were already no warnings nor error messages in French.
Note that this is an error message from the ocamldoc tool, and not an error message from the OCaml compiler itself. Funnily enough, looking that the repository history, this French error message was introduced in the very first version of ocamldoc in 2002 and went undetected (or unreported) for 9 years. Thanks for the historical tidbit!