Site Language: Install translations on the fly.

The language dropdown now includes installed languages and all available translations when the filesystem is writable by WordPress.
Go to wp-admin/options-general.php, select one of the available translations, submit the form and let WordPress handle the rest.
Works for Multisite's Default Language too.

see #29395.

git-svn-id: https://develop.svn.wordpress.org/trunk@30335 602fd350-edb4-49c9-b593-d223f7449a82