application_controller.rb

class ApplicationController < ActionController::Base 
  before_filter :set_locale_from_remote_addr

  private

  def set_locale_from_remote_addr
    return do_something(session[:locale] if session[:locale]

    raise UnknownIPAddessError unless request.remote_addr
    geoip = GeoIP.new(Rails.root + "db/GeoLiteCountry.dat").country(request.remote_addr) 
    raise UnsupportedIPAddressError unless geoip
    
    do_something(geoip.country_code2.downcase.to_sym)
  end

  def do_something locale
    # do something with client's locale
    # for example, set I18n locale
    I18n.locale = locale
    # another example, to set header image by country
    @header_image_filename = "header_image_#{locale}.png"
  end
end