Drupal has a list of languages defined in the function getStandardLanguageList() in LanguageManager.php, based on the list of localise.drupal.org
Instead of this list, we can ship Drupal with a list of all languages defined in ICU, and filter where appropriate. This list can act as the base list for a language picker, useful for content tagging (language of parts), and adding new languages.
Possible solution
We could use the list available in the Intl component of Symfony (http://symfony.com/doc/current/components/intl.html). It uses the language list of ICU (used in Android, iOS etc). These are the languages you will get:
http://source.icu-project.org/repos/icu/icu/tags/release-50-1-2/source/d...
See also
#1314250: Allow filtering/configuration of which languages apply to what (UI, nodes, files, etc)