i18n_controller.rb

class I18nController < ApplicationController
  caches_page :show

  # Renders HTTP cacheable JSON encoded dictionary of translation strings for requested locale and currently selected locale
  def show
    @locale = params[:locale].to_sym
    raise ActiveRecord::RecordNotFound unless I18n.selectable_locales.include?(@locale)
    respond_to do |format|
      format.js do
        @strings = I18n.t('javascript', :locale => @locale)
      end
    end
  end

end

routes.rb

ActionController::Routing::Routes.draw do |map|

  map.i18n '/javascripts/i18n/i18n.:locale.:format', :controller => "i18n", :action => "show"

end

show.js.erb

// Quick and dirty I18n object that echos the basic functionality of the I18n gem.
// e.g. I18n.t('key.to.translate', {interpolation_key: interpolation_value})
var I18n = {
  locale: '<%= @locale %>',
  strings: <%= raw(@strings.to_json) %>,
  t: function(key, interpolations) {
    string = eval("this.strings." + key);
    for (key in interpolations) {
      string = string.replace(new RegExp("\{\{" + key + "\}\}"), interpolations[key])
    }
    return string;
  }
};