I know this is not a Worpdress plugin, but this seemed the most relevant forum to post to.
PoExcel is an Excel Add-In that lets you manage all your (.po & .mo) translations from one Excel file. I hope this tool helps you translating your wordpress, themes and plugins.
For more info: http://www.snippetz.nl/poexcel-manage-your-translations-from-excel/
Regards,
Gerard