iLib
The most comprehensive library of Javascript i18n classes available
Note: development has been moved over to github! Please see https://github.com/iLib-js/iLib
"I18N" means way more than just translation. Other libraries claim to be an i18n solution when translation and string formatting is really all they offer. Yes, iLib has classes that do translation too, but there is also much, much more. From date formatting to name parsing to phone number handling to collation, iLib has classes that do it. And, it has the ability to support thousands of possible locales.
Everything in iLib is written in pure JS with minimal platform dependencies, so it works the same way in many different environments and browsers, from Chrome to Opera to node to Qt/QML. Over 17K unit tests help to guarantee this.
ILib is also the library of choice for enyo apps on webOS which ships on over 10 million LG SmartTVs in over 135 countries. Give it a try and see if it can work for your app too!