Although Drupal 6 handles node translation with the i18n module very well, Ubercart seems to have a bit of difficulty with it. I believe that most of the problems are due to the nature of translated nodes, which are actually completely different nodes that are glued together with some metadata. This fact, for example, is the reason Ubercart will create a product translation as a new product separate from the original.

As I tried to setup a multi-lingual store (English/French) store with Ubercart, I encountered many problems that seemed to be all over Google, but had no resolution posted or the solutions to the problems were fragmented across different sites and support threads. So after a lot of searching, reading PHP backtraces and hacking the Ubercart core modules, I was finally able to have a good Ubercart catalog set up in two languages! I've compiled a list of problems and their solutions below if you are also having problems. Feel free to use any part or all of the code samples on your site.

Note: The code in this tutorial has been confirmed to work with Ubercart 2.2. It should also work with future versions, but may require some minor tweaks.

Problem: "add to cart" and other form buttons need to be translated

Ubercart handles form button texts like "add to cart" via multi-lingual variables. Multi-lingual variables differ from strings that use Drupal's t() function in that they will not appear in the site's Translate Interface utility. Instead, multi-lingual store a value for each of the site's languages and load the appropriate string depending on which language the user is currently browsing in.

In the case of "add to cart", this simply means that one must browse to Administer > Store Administration > Configuration > Product Settings and click on the Edit button. Switch the site language to the desired language and enter new values in the "Add to cart button text" configuration box. Once the values have been saved, you will be able to switch to a different language and enter new values for that language.

Problem: Ubercart does not synchronize product data across translated nodes (product information needs to be updated for each translation)

As I mentioned in the introduction, this is due to the fact that each translation is in fact a node to itself. Since Drupal assigns each node with content type "product" the standard Ubercart attribute set, each product node must be updated individually. Fortunately, there is a quick and easy was to solve this by creating a custom module:

1. Enable the i18nsync ("Synchronize Translations") module in Administer > Site building > Modules
2. Create a custom module which will hook into i18nsync:
   1. Create the folder sites/all/modules/custommod
   2. Paste the following into sites/all/modules/custommod/custommod.info:
      
      name = Custom module
description = A custom module which helps with the translation of an Ubercart store
core = 6.x
   3. Paste into sites/all/modules/custommod/custommod.module:
      

      /**
        * Implementation of hook_i18nsync_fields_alter().
        */
      function custommod_i18nsync_fields_alter($fields, $type) {
        if(in_array($type, uc_product_types())) {
          $fields['uc_products']['#title'] = 'Products';
          // These values were found by doing a print_r($node) in node-product.tpl.php
          $fields['uc_products']['#options'] = array (
            'model' => 'SKU',
            'list_price' => 'List Price',
            'cost' => 'Cost',
            'sell_price' => 'Sell Price',
            'weight' => 'Weight',
            'weight_units' => 'Weight Units',
            'dim_length' => 'Length',
            'dim_width' => 'Width',
            'dim_height' => 'Height',
            'length_units' => 'Length Units',
            'pkg_qty' => 'Quantity',
            'default_qty' => 'Default quantity to add to cart'
          );
        }
      }
      ?>

   (Thanks to these two commenters for posting code updates)

3. Go to Administer > Site building > Modules and enable "Custom module"
4. Go to Administer > Content management > Content types and edit the "Product" content type
   1. Under Workflow Settings > Synchronize translations, select all fields you would like to synchronize. However, do not select the Taxonomy terms option (see below for why)
   2. Repeat this step for any other product classes/content types whose product data you want synchronized

That's all! If you edit a node in one language, you should see the updated product data in all other languages as well. The only caveat is that product attributes and product options will still need to be added and updated on each translation.

Thanks to user gupa on Ubercart support forums for posting this fix (see reference 1)

Problem: Taxonomy terms are untranslated in Catalog block

This one had me puzzled for a long time. At first, I had enabled localized terms for the Catalog vocabulary. After I created some terms, I translated them using the Translate Interface tool as usual, but this did not work. Editing a product would show the localized term in the Catalog list, however viewing the product always resulted in the untranslated term being displayed on the Catalog block. The solution is to navigate to Administer > Content management > Taxonomy and edit the Catalog vocabulary, choosing Per language terms. Use the following procedure to define taxonomy term translations:

1. Browse to Administer > Content management > Taxonomy
2. Add a new term with its translations:
   1. Add a new term to the Catalog vocabulary in the default language of your store and choose that language from the drop-down menu
   2. Repeat the above step, but enter the desired translated texts for the term (including a translated term name)
   3. Repeat the above step, but enter the desired translated texts for the term (including a translated term name)
3. Click on the Translation tab of the taxonomy interface
4. Click Create new translation and pair up the original term with its translations
5. Repeat the step above for each term that you created earlier.

This is why you do not want to synchronize taxonomy terms, with this setup each term is translated the same way nodes are - separate terms per node, linked together with some metadata. Synchronizing taxonomy terms would result in your products appearing in the right categories but in the wrong languages!

Problem: Adding items to the cart in one language, then switching to another language does not localize the cart contents

This is an unusual situation, but it's a very annoying problem; adding an item to the cart, switching languages and then click on that same item in the cart will open the product in first language, not the one the customer is browsing your website in. To solve this, hook_cart_item() will be used to override the default cart rendering with our own version which will replace node with translated nodes. Open the custom module file, sites/all/modules/custommod/custommod.module, which was created earlier and add before the closing ?> tag:

/**
  * Implementation of hook_add_to_cart().
  */
function custommod_add_to_cart($nid, $qty, $data) {
  /* Due to Drupal's use of multiple nodes for product translations, the same
   * product in a different language is treated as a different product entirely.
   * This is problematic as the same product in different languages can be added
   * to the cart simultaneously. This function works around that problem by
   * always using the tnid/original node. As a result, the cart must be
   * localized as it is displayed.
   */
  $node = node_load($nid);
  // Determine if this node is the source node or a translated one
  // Remember: tnid is 0 if there are no translations
  $is_source = ($node->nid == $node->tnid || $node->tnid == 0) ? 1 : 0;
  if ($is_source) {
    // If it is the source, then all is well…
    $result[] =  array('success' => TRUE);
  } else {
    /* If we are not the source node, then fail to add this product silently and
     * call uc_cart_add_item() to add the source node's product instead. It will
     * be localized later - see custommod_cart_item()
     */
    uc_cart_add_item($node->tnid, $qty, $data);
    $result[] = array('success' => FALSE, 'silent' => TRUE);
  }
  // Remember: We need an array in an array here
  return $result;
}

/**
  * Implementation of hook_cart_item().
  */
function custommod_cart_item($op, &$item) {
  /* hook_cart_display() isn't really a hook, it's mostly for internal use.
   * However, we do need to access later. Setting $item->module forces
   * a module_invoke() call in uc_cart.module to call custommod_cart_display()
   * instead of the default uc_product_cart_display(). We will call
   * uc_product_cart_display() inside our function to ensure things work as
   * usual in future versions.
   */
  $item->module = "custommod";
  /* Note that although it is possible to use check for case 'load' in $op and
   * then override the $item->nid and $item->title values, this will cause bugs
   * when attempting to add or remove products in different languages. To
   * resolve these bugs, we are forcing the use of custommod_cart_display() and
   * rewriting the code for the title, img, and anchors to localize the cart.
   */
}
/**
  * Implementation of hook_cart_display().
  */
function custommod_cart_display($item) {
  /* Call uc_product_cart_display() to get things setup as usual and to ensure
   * this hack still works even if uc_product_cart_display changes at some point
   * in the future.
   */
  $display_item = uc_product_cart_display($item);
  // Get the translations, if any.
  $node = node_load($item->nid);
  global $language;
  $translations = translation_node_get_translations($node->tnid);
  if ($translations[$language->language]) {
    // Reminder: NEVER override the nid. That is what causes the bugs!
    $tnode = node_load($translations[$language->language]->nid);
    $display_item["title"]["#value"] = node_access('view', $tnode) ? l($tnode->title, 'node/'. $tnode->nid) : check_plain($tnode->title);
    $display_item["image"]["#value"] = uc_product_get_picture($tnode->nid, 'cart');
  }
  return $display_item;
}
?>

After reloading the Custom module at Administer > Site building > Modules, the cart should behave properly when switching languages. As well, adding a product to the cart in one language, switching languages, then adding it again should not result in two different products being added to the cart. Instead, the quantity of the product will increase by 1.

Credit for this solution goes to user Docc at Ubercart forums, who posted the code sample (see reference 3)

Problem: Ubercart does not localize products during checkout

This one was a bit trickier to solve, since there's no real elegant way to trick ubercart into localizing the products while using the standard checkout pane. As a result, we will have to disable the stock checkout pane and use the replacement provided by the code below instead (the replacement pane is called "Your order").

/**
  * Implementation of hook_checkout_pane().
  */
function custommod_checkout_pane() {
  /* Replacement for standard cart contents pane. Although hook_cart_item() can
   * be used to localize the checkout pane, then we get into trouble while
   * trying to localize the cart display (see the comments above). The best way
   * that I can think of to work around this is to disable the stock cart
   * contents pane and enable this one instead.
   */
  $panes[] = array(
    'id' => 'custommod_cart',
    'callback' => 'custommod_checkout_pane_custommod_cart',
    'title' => t('Your order'),
    'desc' => t('Display the (localized) contents of a customer\'s shopping cart.'),
    'weight' => 2,
    'process' => TRUE,
    'collapsible' => FALSE,
  );
  return $panes;
}

/**
  * Callback for our implementation of hook_checkout_pane()
  */
function custommod_checkout_pane_custommod_cart($op, &$arg1, $arg2) {
  /* The code below is copied from uc_checkout_pane_cart() and is slightly
   * modified to localize the cart contents before displaying it. If you are
   * using this, you need to keep an eye on uc_checkout_pane_cart() to make sure
   * that if there is an important change or bugfix, you make the same change
   * here.
   */
  switch ($op) {
    case 'view':
      $contents['cart_review_table'] = array(
        '#value' => theme('cart_review_table'),
        '#weight' => variable_get('uc_pane_cart_field_cart_weight', 2),
      );
      return array('contents' => $contents, 'next-button' => FALSE);
    case 'review':
      $items = uc_cart_get_contents();
      $output = '';
      $context = array(
        'revision' => 'themed',
        'type' => 'cart_item',
        'subject' => array(),
      );
      global $language;
      foreach ($items as $item) {
        $node = node_load($item->nid);
        $translations = translation_node_get_translations($node->tnid);
        if ($translations[$language->language]) {
          $tnode = node_load($translations[$language->language]->nid);
        } else {
          $tnode = $node;
        }
        $desc = check_plain($tnode->title) . uc_product_get_description($item);
        $price_info = array(
          'price' => $item->price,
          'qty' => $item->qty,
        );
        $context['subject'] = array(
          'cart' => $items,
          'cart_item' => $item,
          'node' => $tnode,
        );
        $output .= '';
      }
      $output .= '
'. $item->qty .'×
'. $desc
                  .'
'. uc_price($price_info, $context) .'
';
      $review[] = $output;
      return $review;
  }
  return $result;
}
?>

Thanks to user totsubo at Ubercart forums for helping me test this part of the module!

See also

• The sequel to my Ubercart i18n adventures

References:

1. Information about localized taxonomy in thread Multiple language Ubercart website on the Ubercart support forums
2. Lots of good information at a thread titled i18n issues i D6/UC2 for Multilingual sites on the Ubercart support forums
3. Method for product synchronization was based on a small code sample by Docc at Drupal node #456358