> i18n-js is a js library, which means you have to do the formatting in your js code, then passing the formatted data into the template engine where you have the placeholders for them

Not sure I follow, taken from a React component:

    Component = React.createClass                                                
      render: ->                                                                
        @div null,
          @t('welcomeMessage', username: @props.CurrentUser.displayName)