localized_attributes.rb

class Post < ActiveRecord::Base
  validates_presence_of :title_en, :title_es

  def localized_attribute_for(name, locale = I18n.locale)
    "#{name}_#{locale}"
  end

  private

  def method_missing(name, *args)
    attribute = localized_attribute_for name
    send attribute, *args if respond_to?(attribute)
  end

  def respond_to_missing?(name)
    respond_to? localized_attribute_for(name)
  end
end

class ApplicationController < ActionController::Base
  before_action :set_locale

  protected

  def set_locale
    I18n.locale = params[:locale] || I18n.default_locale
  end
end

class PostsController < ApplicationController
  def index
    @posts = Post.all
  end

  def show
    @post = Post.find params[:id]
  end
end