Install: Don't ask for the user's language a second time if they chose English originally.

see #28577.

Built from https://develop.svn.wordpress.org/trunk@29058


git-svn-id: http://core.svn.wordpress.org/trunk@28846 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Unload the current default textdomain in wp_install_load_language().

see #28577.

Built from https://develop.svn.wordpress.org/trunk@29059


git-svn-id: http://core.svn.wordpress.org/trunk@28847 1a063a9b-81f0-0310-95a4-ce76da25c4cd
4.0-beta1.

Built from https://develop.svn.wordpress.org/trunk@29060


git-svn-id: http://core.svn.wordpress.org/trunk@28848 1a063a9b-81f0-0310-95a4-ce76da25c4cd