I had a similar problem. It turned out that i made a typo in store_controller.erb. In the index action, instead of 
if params[:set_locale]
I wrote 
if params[:locale]
and couldn’t select Espanol.
I’ve had issues with the language box in Safari 5, I can’t select Espanol. the code for that is in: store.html.erg 

<%= select_tag 'locale', options_for_select(LANGUAGES, I18n.locale), :onchange => 'this.form.submit()' %>
Anyone knows the answer?
On mine (Rails 2.3.8) I had to add .to_s to I18n.locale as it returns a symbol, not a string. 

<%= select_tag 'locale', options_for_select(LANGUAGES, I18n.locale.to_s), :onchange => 'this.form.submit()' %>
Pierre says:

This refers to Playtime Task 2, Activity 2

I have added a helper to application_helper.rb 

  def currency_to_locale(price)
    price = price * 1.3 if 'es' == I18n.locale.to_s
    number_to_currency price
  end
Then in app/views/store/index.html.erb I utilize the helper 

  ...
  currency_to_locale(product.price) %>
  ...
And also in app/views/store/_line_item.html.erb 

  ...
  <td class="item_price"><%= currency_to_locale(line_item.total_price) %></td>
  ...
And finally in app/views/carts/_cart.html.erb 

  ...
  <td class="total_cell"><%= currency_to_locale(cart.total_price) %></td>
  ...
This works fine in the browser. But the functional and integration test fails each with the same error 

test_shipping_the_product(UserStoriesTest):
ActionView::Template::Error: undefined method `currency_to_locale' for #<#<Class
:0x00000004882c88>:0x00000004990b70>
    ~/work/depot/app/views/line_items/_line_item.html.erb:9:in `_app_
views_line_items__line_item_html_erb___4544981081009683343_38610780'

A solution to the problem can be found here

You have to add following to test/test_helper.rb 

  require './app/helpers/application_helper'
  require ApplicationHelper

Then the test should run without errors