http://cvsweb.netbsd.org/bsdweb.cgi/src/lib/libc/nls/C.msg
and sysinst (8)
http://cvsweb.netbsd.org/bsdweb.cgi/src/distrib/utils/sysinst/msg.mbr.en
http://cvsweb.netbsd.org/bsdweb.cgi/src/distrib/utils/sysinst/msg.mi.en
http://cvsweb.netbsd.org/bsdweb.cgi/src/distrib/utils/sysinst/arch/i386/msg.md.en
This is about i386. See other architectures.