Accept and store translations data from the plugin and theme update check endpoints.

Send site locale. Rename wp_get_installed_language_data() to wp_get_installed_translations().

see #18200.

Built from https://develop.svn.wordpress.org/trunk@25536


git-svn-id: http://core.svn.wordpress.org/trunk@25456 1a063a9b-81f0-0310-95a4-ce76da25c4cd