Ruby on Rails I18n good practice

Following is example of what I think its good practice on setting up I18n on pages, this mini pattern can later grow but I think it is a good starting point.

Let's say you have BaseController which will be extended by your controllers.

class BaseController < ActionController::Base
  
  before_filter :set_session_locale

  def set_session_locale
    session[:selected_locale] = params[:lang].nil? ? session[:selected_locale] : params[:lang] 
    
    I18n.locale = session[:selected_locale] || I18n.default_locale
  end

end

before_filter will make sure that this is triggered before each request, this is important since I18n will not persist beyond one request. Priority will be given to URL param lang, if not than to session locale, if not than to default locale.

Session will be used to propagate locale selected. You can force language with each request to each controller than extends BaseController for example:

localhost:3000/index?lang=de

To extend this approach for registered users you can use another controller which will be used once user is logged in. Here is example of this ApplicationController

class ApplicationController < BaseController

  before_filter         :set_user_locale
  skip_before_filter    :set_session_locale
  
  # Application code which will among other things set @user

  def set_user_locale
    I18n.locale = @user.locale|| I18n.default_locale
  end

end

We can see now that before_filter is used to trigger user preferred locale, if none is present it default to I18n one.

I18n.locale = @user.locale|| I18n.default_locale

We can see that skip_before_filter is used to disable BaseController's set session 

locale, since priority is now to user. If we didn't added this filter this would still work fine, 

but there would be unnecessary call to set_session_locale method, from  

ApplicationController.

Have fun and tweak approach to your needs. If you have any questions leave it at comments section I will be happy to answer them.