ResX Translation Helper has the following features:

• Online Tool ( = no need to install anything. Runs on every browser)
• Authentication (only registered users are allowed to translate)
• Easy to use (for both administrator and translators)
• Automatic backup (each file will be copied to a backup folder as soon as the translator makes any modification)
• Automatic FTP upload (nothing to do for the admin after translation. Additionally translator could check his changes on the fly!)
• Free to use 🙂

It’s an open source solution hosted on http://resx.Codeplex.com. You will also find installation details there. Additionally you might have a look at the article published in german magazine dotnetpro 05/2011.