Updating fields using the wrapper fails if locale is enabled

I found out of a similar problem, when using auto_nodetitle. There tokens are not provided for node creation, but for node updates.

As I recognized this occurs because in entity_metadata_field_property_get() the $langcode is retrieved as 'und', but the node form provided its fields as (in my case) 'de' (for German).

So I have an entity with

$entity->field_xy = array(
  'de' => array(
    0 => array('nid' => 3)
  ),
);

but the due to the $langcode it wants to look for 'und'.

Btw. $options['language'] is set to NULL, as $this->getPropertyLanguage() returns NULL, in EntityStructureWrapper::getPropertyValue(), where entity_metadata_field_property_get() is called.

By further investigation, I guess I fell into a core bug that leeds to some wrong field language associations: #1238042: Field display languages are cached before locale has changed field language.. i recognized that after testing to patch the entity wrapper with some defaultLanguage behaviour.
The patch for that is attached here.

Looking into core, the only entity to providing a language property seems to be 'node'. I guess simply looking for $data->language would not fit to other entity approaches, but therefore I'm not enough into the entity api, to provide a more generic patch. Hope this patch might help anyway.

Related issue: #1260640: Improve field language API DX.