Translation set not being saved properly on the product entity

After creating a couple of products, the issue appears worse, still. I now encountered loss of form data again (the values entered in the translatable fields) and a NOTICE is thrown:

'Notice: Undefined property: stdClass::$field_mytranslatable_field in commerce_product_ui_product_form_translation_submit() (line 540 of /commerce/modules/product/commerce_product_ui.module).'

here is the related code from commerce_product.ui.module:

<?php

/**
* Submit callback for commerce_product_ui_product_form().
*
* Checks if translation is enabled for the product and handles language changes.
* Since this handler may change the language of submitted form values it should
* be the first submit handler called.
*
* @see commerce_product_ui_form_commerce_product_ui_product_form_alter()
*/
function commerce_product_ui_product_form_translation_submit($form, &$form_state) {
  // Get an array of available languages.
  $available_languages = field_content_languages();
  list(, , $bundle) = entity_extract_ids('commerce_product', $form_state['commerce_product']);

  foreach (field_info_instances('commerce_product', $bundle) as $instance) {
    $field_name = $instance['field_name'];
    $field = field_info_field($field_name);
    $previous_language = $form[$field_name]['#language'];

    // Handle a possible language change; new language values are inserted and
    // the previous values are deleted.
    if ($field['translatable'] && $previous_language != $form_state['values']['language']) {
// ##### LINE 540: ######
      $form_state['values'][$field_name][$form_state['values']['language']] = $form_state['commerce_product']->{$field_name}[$previous_language];
      $form_state['values'][$field_name][$previous_language] = array();
    }
  }
}

looking at the assignment in line 540 with a debugger tells me this line equates to the pseudo statement below:

$form_state['values'][$field_name][$form_state['values']['language']] = $form_state['commerce_product']->{$field_name}[$previous_language];

$form_state['values']['field_test']['fr'] = [0]['value']['my translatable field value (text entered)'];

...that makes perfect sense to me - the index is undefined as it should be added... however slightly rephrasing the line does away with the loss of the value and the "undefined index" notice:

$temp = $form_state['values'][$field_name];
      $temp[$form_state['values']['language']] =  $form_state['commerce_product']->{$field_name}[$previous_language];
      $form_state['values'][$field_name] = $temp;

The issue with the translation not being registered as such as described in the first post still stands.

edit2:

On a different install base, $form_state['commerce_product']->{$field_name} is not defined.
My go at this would be to replace line 540 as follows:

old:

$temp[$form_state['values']['language']] =  
    $form_state['commerce_product']->{$field_name}[$previous_language];

new:

$form_state['values'][$field_name][$form_state['values']['language']] = 
    $form_state['values'][$field_name][$previous_language];

this seems to do away with the NOTICE and preserve the fields content (translation set still being falsly set to english though). Any downsides to this?

miro_dietiker was kind enough to help me narrow down the issue:
the entity is always saved in the site's default language, ignoring the select value in the product creation form.
other language variables (such as the user/UI language setting) do not seem to have any (unwanted) influence here.

I've made some progress, narrowing in on the cause by stepping through each function invoked when saving the product:

the selected language is passed on all the way, until it gets screwed over by /entity_translation/includes/translation.handler.inc:345 where the default language suddenly pops up in the debuggers var list:

  public function initTranslations() {
    $langcode = $this->getLanguage(); // returns the sites default language

    if (!empty($langcode)) {
      $translation = array('language' => $langcode, 'status' => (int) $this->getStatus());
      $this->setTranslation($translation);
      $this->setOriginalLanguage($langcode); // sets the sites default language as original language pre-save.
    }
  }

I guess there lies the problem. Instead of using the language passed along with the entity properties, the original language is always set to be the sites default. Thus the issue is probably not so much related to commerce, as it is to entity_translation.

response by plach @ #1420572: setOriginalLanguage ignores language set in entity

ET knows nothing about entity-specific details: Commerce needs to override the DefaultEntityTranslationHandler::getLanguage() method to return the proper value.

commerce defines a custom translation handler, but according to my debugger, the code is never actually run when saving or updating commerce_product entities.

can someone could please verify this issue?

small patch, huge relief.
thank you ever so much!