CanJS and messageformat.js example

en.json

{
    numMessages: "You have {numMessages, plural, one {one message} other {#{numMessages} messages}}."
}

i18n.js

steal("en.json", "messageformat.js", function( translation ) {

    var mf = new MessageFormat("en"),
        cache = {};

    // Add EJS helper
    can.extend( can.EJS.Helpers.prototype, {

        message : function( key, data ) {
            if ( ! cache[ key ] ) {
                cache[ key ] = mf.compile( translation[ key ] || "ERROR: <" + key + "> NOT DEFINED" );
            }

            return cache[ key ].call( this, data || {} );
        }

    });

});

inbox.ejs

<h3><%= message( "numMessages", { numMessages: 10 }); %></h3>