How to create the path anyway for untraslated content?

Thank you for your answer, I'm actually trying to follow the first way: create a path alias for all languages when creating a node.

So here's what I did:

function helper_node_insert($node) {
  // @todo Remove the next line when http://drupal.org/node/1025870 is fixed.
  unset($node->uri);
  _helper_manage_alias_for_untraslated_nodes($node, 'insert');
}

function helper_node_update($node) {
  _helper_manage_alias_for_untraslated_nodes($node, 'update');
}

function _helper_manage_alias_for_untraslated_nodes($node, $op) {
  // Retrieve all languages already translated for this node:
  $existing_translations = $node->translations->data;

  // Retrieve all active languages in the site:
  $available_languages = language_list();

  // Diff between the above arrays: the results is a list of languages not yet translated:
  $pending_translations = array_diff_key($available_languages, $existing_translations);

  module_load_include('inc', 'pathauto');
  $uri = entity_uri('node', $node);

  if( !empty($pending_translations) ) {
    // add the alias for the untraslated languages:
    foreach( $pending_translations as $pending_translation ) {
      pathauto_create_alias('node', $op, $uri['path'], array('node' => $node), $node->type, $pending_translation->language);
    }
  }
}

The code seems to works nice, please tell me if it's a good stuff or there are better ways to proceed.

The only thing I haven't resolved yet is deletion management: we have 2 cases here:

1. The SOURCE node is deleted -> all alias have to be deleted (this already works by default)
2. A translations is deleted -> its alias have to be kept

To resolve point 2 I've tried:

function helper_node_delete($node) {
  // Only act on a translation, not a source:
  if ( $node->translations->original != $node->language ) {
    _helper_manage_alias_for_untraslated_nodes($node, 'insert');
  }
}

or:

function helper_entity_translation_delete($entity_type, $entity, $langcode) {
  module_load_include('inc', 'pathauto');
  $uri = entity_uri($entity_type, $entity);
  pathauto_create_alias('node', 'insert', $uri['path'], array('node' => $entity), $entity->type, $langcode);
}

But both doesn't work: alias of deleted translation is deleted too.

Any suggestion?

Thank you very much

@barami: this is very interesting: thank you very much!

I already thought to create a "parallel" language neutral path in any case, but abandoned the idea because I was not sure if this might have some kind of consequence in path/node management. Another reason is that different tokens for different path languages can't works with a unique UND path (with my partial solution above you can setup pathauto patterns with different tokens for every language).

But reading the link you have suggested (and the derivated ones) seems it could be a reasonable solution.