The prompts come from PAM, so you'll have to edit the source code to change their wording.
user@host:/tmp/pam-1.1.8$ grep '(current) UNIX password:' -B2 -A3 modules/pam_unix/pam_unix_passwd.c
retval = _unix_read_password(pamh, lctrl
,Announce
,_("(current) UNIX password: ")
,NULL
,_UNIX_OLD_AUTHTOK
,&pass_old);
user@host:/tmp/pam-1.1.8$ _