I've used https://pypi.python.org/pypi/poxls/ in client projects successfully, but if you want to contribute to Plone, familiarize with .po files, it's quite trivial
I thought the Plone project on Transifex was just a try-out and is outdated. A colleague used it last year for Dutch Plone translations and we had trouble merging a download of his translations, because lots had changed in plone.app.locales since the last time it was updated on Transifex.
But I would love to be wrong.