One answerer suggested XML, but that would be very heavyweight for shipping to the client.
Consider doing something similar to the minification process of JavaScript. Take your strings that you are using on the server/C# side and process your JavaScript files using something like Perl/Python/Ruby, replacing the ids of the strings with the strings themselves.
So, if your javascript says:
alert($MY_TEXT$)
it gets changed to
alert("The string associated with $MY_TEXT$")
The Perl/Python/Ruby process takes place in your "build" of the JavaScript files, before deployment to your production servers.