The idea is to allow to export messages that have no accepted translation for a given locale for a given product version.

These keys then can be given to someone else, e.g. a professional translations company to translate.

I think we should, at least at first, build this as a admin feature. Add an export section to the admin interface, and allow the user to select language and product version, then click export and have a zip file with properties file sent (streamed without being loaded into memory as a whole) to the browser.