#compdef -value-,LANG,-default- -value-,LANGUAGE,-default- -P -value-,LC_*,-default-

local expl locales

if (( $+commands[locale] )); then
  locales=( $(_call_program locales locale -a) )
  [[ $OSTYPE = *-gnu ]] && locales=( ${locales/utf8/UTF-8} )
else
  locales=( /usr/lib/locale/*(:t) )
fi

_wanted locales expl locale compadd -a "$@" - locales