We have already https://pypi.python.org/pypi/collective.ttwpo which I created for a customer recently, see also: New add-on collective.ttwpo
It can be extended with little effort to also override existing po-files.
Currently one can upload po-files or fetch them from a third party service (zanata is supported, others need adapters).
Missing: A simple TTW-Gettext-Editor inside Plone, for those not in the mood to use Zanata (or other services).
Also the current UI is very simple and for sure there is room for improvements.