I see only English (United States) and pl_PL (this language pack was present in 3.9.2 before upgrade to 4.0) options in language select. It looks like the language pack must be downloaded before trying to select this particular language... for example I've downloaded de_DE package and it showed in language select. Andrew Nacin wrote:

"2. You should be able to choose/switch a language from the general settings screen, after which the language pack should be downloaded." (http://make.wordpress.org/core/2014/07/02/internationalization-project-updates/).

Am I missing something?