msghack

This very special tool can swap msgid and msgstr in a PO file, remove all translations in order to convert a translation file into a template file, and can also append the contents of one PO file to another.

It is part of Red Hat Linux 9, but hasn't been seen compiled for Windows yet. It also doesn't have a man-page on Red Hat Linux, but here you can see the help when running msghack --help:

Usage:  /usr/bin/msghack  [OPTION] file.po [ref.po]
This program can be used to alter .po files in ways no sane mind would think about.
      -o                result will be written to FILE
      --invert          invert a po file by switching msgid and msgstr
      --master          join any number of files in a master-formatted catalog
      --empty           empty the contents of the .po file, creating a .pot
      --append          append entries from ref.po that don't exist in file.po

Note: It is just a replacement of msghack for backward support.