Sometimes it can be useful to set your application locale based upon the request IP. You can easily achieve this using Geocoder. Among the many things Geocoder does, it can also tell the location of a request.

First, add Geocoder to your Gemfile

# Gemfile
gem 'geocoder'

Geocoder adds location and safe_location methods to the standard Rack::Request object so you can easily look up the location of any HTTP request by IP address. You can use this methods in a before_action in your ApplicationController:

class ApplicationController < ActionController::Base
  before_action :set_locale_from_request

  def set_locale_from_request
    country_code = request.location.data["country_code"] #=> "US"
    country_sym = country_code.underscore.to_sym #=> :us

    # If request locale is available use it, otherwise use I18n default locale
    if I18n.available_locales.include? country_sym
      I18n.locale = country_sym
    end
end

Beware that this will not work in development and test environments, as things like 0.0.0.0 and localhost are valid valid Internet IP addresses.

Limitations and alternatives

Geocoder is very powerful and flexible, but needs to be configured to work with a geocoding service (see more details); many of which place limits on usage. It’s also worth bearing in mind that calling an external service on every request could impact performance.

To address these, it can also be worth considering:

1. An offline solution

Using a gem like GeoIP (see here) allows lookups to happen against a local datafile. There may be a trade-off in terms of accuracy, as these datafiles need to be kept up-to-date.

2. Use CloudFlare

Pages served through CloudFlare have the option of being geocoded transparently, with the country code being added to the header (HTTP_CF_IPCOUNTRY). More detail can be found here.