Dealing with node language when manually updating node
Yes - it is safe to assume this. The locale module will deal with this by setting it to the language code of the default language, ex 'en'.

Citing from: http://drupal.org/update/modules/6/7#drupal_language_none

In Drupal 7, the LANGUAGE_NONE constant should be used for nodes or other items that do not have a specific language associated with it.

http://api.drupal.org/api/drupal/includes--bootstrap.inc/constant/LANGUAGE_NONE/7