POSIX locale and localedef don't support NLS_<anything> - there is an NLSPATH that is part of locale settings. With the exception of LANG, the locale variables all are LC_<something>. The NLS_<anything> variables I've seen have been in support of databases.

You will want to follow Vino's advice and use the LANG environment variable, but leave the NLS_ stuff alone. It doesn't affect locale.