After having helped translating another game using this site
http://translatewiki.net/wiki/Main_Page, I would suggest that you take a look at it for this project as well.
The Wiki has support for handling po files directly, and gives a very nice interface to handle translations.
I understand that it will require some work, and won't give the same overview of how the translation works in the "bigger" picture.
But it handles all translations big and small the same way.
There will be no need to upload po files from each translator.
Multiple translator can work at the same time, and any collision will be detected or even handled automatically.
Fuzzy message are easily detected and can be fixed quickly.
The wiki also shows other translations in other languages which might help you. E.g. as a danish translator I get swedish and norwegian translations, which can be quite helpful.
It also support the Comment language qq, so translators/developers can add notes to the text being translated.
