Ruby on Rails i18n: using user locale

add_locale_to_users.rb

# db/migrates/*_add_locale_to_users.rb
class AddLocaleToUsers < ActiveRecord::Migration
  def change
    add_column :users, :locale, :string, default: "fr"
  end
end

application_controller.rb

# app/controllers/application_controller.rb
class ApplicationController < ActionController::Base
  protect_from_forgery with: :exception
  before_action :set_current_user
  before_action :set_locale

  private
  
  def set_current_user
    if session[:user_id]
      @current_user = User.find(session[:user_id])
    end
  end
  
  def set_locale
    if @current_user.try(:locale)
      I18n.locale = current_user.locale
    end
  end
end