Review use of enabled/disabled languages with language_list(), etc.

Drupal uses its language list sometimes with all languages considered, sometimes not all of them. In http://drupal.org/node/1387608#comment-5469204 @sun pointed out we should default language_list() to return the enabled language list only, however, most places which call it in core consider the full list, not just enabled languages. We should review the places it is used (about 55 calls as per that thread) whether they need the whole list or just enabled languages. Drupal has a bit of an identity misunderstanding at some places as to what does disabling a language mean. So this would be great to uncover problem areas and find solutions.

Postponed of #1387608: Unify language_list() and locale_language_list() being committed.