Dariusz Suchojad <dsuch(at)zato(dot)io> writes:
> The thing I'm struggling with is how to create a test environment to
> reproduce this situation in.
> The goal is not to store data in French in the tables, rather how to
> make Postgres output its own messages in French.
It looks like you set the locale settings properly. I'm guessing that
the build lacks localization support. Check the output of
"pg_config --configure" to see if --enable-nls is mentioned. If it
isn't, you need to rebuild with that configuration option. If it is,
then my only remaining guess is that the French message files for
postgres aren't installed.
regards, tom lane