Ocaml-gettext program can be used just as any OCaml program. The only difference with standard OCaml program is that they come with a bunch of command line options which are specific to OCaml program. In most cases, you just have to define a well suited LC_ALL or LANG environment variable. Since ocaml-gettext is compatible with gettext, if your environment variable works with gettext, it should also works with ocaml-gettext.

You can find more details in the gettext documentation or in the ocaml-gettext-options manpage.