Set I18N locale based on HTTP_ACCEPT_LANGUAGE header.

application_controller.rb

class ApplicationController < ActionController::Base
  # ...
  
  before_filter :set_locale
  
  # ...

  def set_locale
    logger.debug "* Accept-Language: #{request.env['HTTP_ACCEPT_LANGUAGE']}"
    I18n.locale = locale_string(locale) if locale
    logger.debug "* Locale set to '#{I18n.locale}'"
  end

  private

  def locale
    @local ||= locale_from_param || locale_from_header
  end

  def locale_hash(local_string)
    locale, qvalue = local_string.split(';q=')
    language, territory = locale.split('-')
    {
      language: language.downcase,
      territory: territory ? territory.upcase : nil,
      qvalue: qvalue ? qvalue.to_f : 1.0
    }
  end

  def locale_string(locale_hash)
    language = locale_hash[:language].downcase
    territory = locale_hash[:territory].upcase if locale_hash[:territory]
    territory ? "#{language}-#{territory}" : language
  end

  # convert a locale hash into an available locale hash.
  # if this app supports the given locale, returns the locale.
  # if this app supports not the exact locale but an alternate locale with the same language,
  # returns the alternate locale.
  # otherwise, returns nil.
  def available_locale(locale)
    @@available_locales ||= ['en', 'ja'].map{|l| locale_hash(l) }
    same_languages = @@available_locales.select{|l| l[:language] == locale[:language] }
    exact_sames = same_languages.select{|l| l[:territory] == locale[:territory] }
    exact_sames.empty? ? same_languages.first : exact_sames.first
  end

  def locale_from_param
    return nil unless params[:locale]
    locale = locale_hash(params[:locale])
    available_locale(locale)
  end

  def locale_from_header
    http_accept_language = request.env['HTTP_ACCEPT_LANGUAGE']
    return nil unless http_accept_language
    locales = http_accept_language.gsub(/\s+/, '').split(/,/).map{|l| locale_hash(l) }
    # sort locale hashes by qvalue. to make it descending order, reverse it.
    sorted_locales = locales.sort{|a, b| a[:qvalue] <=> b[:qvalue] }.reverse!
    filtered_locales = sorted_locales.map{|l| available_locale(l) }
    filtered_locales.first
  rescue => e
    nil
  end
end