action "_core_ updated successfully" to execute code after core update ?

I discovered a very interesting new action in WordPress 3.3 : “_core_ updated successfully”. However I am troubled by the leading underscore which (I suppose) is used to keep this action private.

I am interested by this action as it would be a convenient way to automatically update languages files on multilingual sites (I believe that WordPress does update only the languages files included with localized distributions. This is OK for monolingual sites but not for multilingual sites).

My questions :
Why would the WordPress team keep this action private ?
Do you know an alternative way to do execute code after a WordPress core update ?