localized_posts.rb

class Post < ActiveRecord::Base
  validates_presence_of :locale
  validates_inclusion_of :locale, in: %w(en es)

  def self.with_locale(locale)
    where locale: locale
  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 = scope
  end

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

  private

  def scope
    Post.with_locale I18n.locale
  end
end