blog_prepend_i18n_path.rb

class BlogPrependI18nPath < Middleman::Extension
  option :blog_name, 'blog'

  def after_configuration
    manipulator = Manipulator.new(@app, options)
    @app.sitemap.register_resource_list_manipulator(:"fix_i18n_article_paths_#{options.blog_name}", manipulator)
  end

  class Manipulator
    def initialize(app, options)
      @app = app
      @options = options
    end

    def manipulate_resource_list(resources)
      blog.data.articles.each do |article|
        language_path = i18n.path_root(article.locale).gsub(%r{^/}, '')
        destination_path = Pathname(language_path).join(article.destination_path).to_s
        article.destination_path = destination_path
      end
      resources
    end

    private

    def blog
      @blog ||= @app.extensions[:blog].values.find { |i| i.options.name == @options.blog_name }
    end

    def i18n
      @i18n ||= @app.extensions[:i18n]
    end
  end
end

::Middleman::Extensions.register(:blog_prepend_i18n_path, BlogPrependI18nPath)