This lets middleman's `url_for` see the locale parameter even if `relative_url` option is true.

middleman_ext.rb

# coding: utf-8

module MiddlemanExt
  module Util
    def self.included(base)
      base.send :include, ClassMethods

      base.send :alias_method, :url_for_without_locale, :url_for
      base.send :alias_method, :url_for, :url_for_with_locale
    end

    module ClassMethods
      # NOTE: This assumes:
      #     - set relative_links true
      #     - activate :relative_assets
      def url_for_with_locale(app, path_or_resource, options={})
        _opt = options
        _opt.symbolize_keys!

        # To set locale manually, remove the key from options
        locale = _opt.delete(:locale) || I18n.locale.intern

        # Handle Resources and other things which define their own url method
        # (* borrowed from middleman)
        url = path_or_resource.respond_to?(:path) ? "/#{path_or_resource.path}" : path_or_resource
        url = url.gsub(' ', '%20')

        # Locale prepend target is which begins '/',
        # because if a relative path was specified,
        # its locale should be prepended to current_page.url
        # NOTICE:
        #   In cases below also are not concerened:
        #     - if url is made from the only fragment.
        #     - if url includes the protocol prefix.
        if url =~ /^\//
          # If it is requested default locale,
          # locale prefix for url is blank.
          if I18n.default_locale == locale
            # If current locale is not the default locale
            unless I18n.locale == I18n.default_locale
              # url might have a prefix of current locale,
              # so it should be take away.
              url.sub!(/^\/#{I18n.locale}/, '')
            end
            # If current locale is default locale,
            # url might not have locale prefix.
            # ( is that so ?)

          else
            # requested locale is not a default locale,
            # so we should append the locale prefix to the url,
            # if it does not have.
            if url !~ /^\/#{locale}/
              url = "/#{locale}#{url}"
            end
          end
        end

        url_for_without_locale(app, url, _opt)
      end
    end
  end
end

module Middleman
  module Util
    class << self
      include MiddlemanExt::Util
    end
  end
end