Don't forget to edit config/database.yml first.

Terminal:

script/plugin install \
    http://svn.globalize-rails.org/svn/globalize/globalize/trunk 
rake globalize:setup 

…in config/environment.rb:

include Globalize
Locale.set_base_language 'en-US'

…in the app controller, before any other filter:

  before_filter :set_locale
  # FIXME, NOT TESTED YET, PROBABLY DOESN'T WORK.
  def set_locale
    if params[:locale]
      # The user has explicitly requested a locale.
      session[:locale] = params[:locale]
      Locale.set params[:locale]
    elsif session[:locale].empty?
      # The user has a current session.
      Locale.set session[:locale]
    elsif ! request.env["HTTP_ACCEPT_LANGUAGE"].empty?
      # We use the browser's locale settings.
      Locale.set request.env["HTTP_ACCEPT_LANGUAGE"][/[a-z]{2}-[a-z]{2}/i]
    else
      # the default locale defined in config/environment.rb is used.
    end
  end

…in the model:

class Blog < ActiveRecord::Base
  translates :title, :post
end