Allow a language to be chosen before installing WordPress. First pass. 

* Checks WordPress.org for available languages.
* In get_locale(), starts using the WPLANG option that has existed in multisite since the MU days.
* Adds new argument to wp_install() for setting WPLANG.

see #28577. 

Built from https://develop.svn.wordpress.org/trunk@28774


git-svn-id: http://core.svn.wordpress.org/trunk@28587 1a063a9b-81f0-0310-95a4-ce76da25c4cd