Language packs: No WPLANG anymore.

* The WPLANG constant is no longer needed. Remove define('WPLANG', ''); from wp-config-sample.php. Populate WPLANG option based on the WPLANG constant. When get_option('WPLANG') is an empty string it will override WPLANG.
* Introduce translations_api() which is available to communicate with the translation API. Move translation install related functions to a new file.
* Replace mu_dropdown_languages() with wp_dropdown_languages(). wp_dropdown_languages() is now populated by the translation API.
* Remove wp_install_load_language() and allow load_default_textdomain() to switch a core translation.

fixes #13069, #15677, #19760, #28730, #29281. 
Built from https://develop.svn.wordpress.org/trunk@29630


git-svn-id: http://core.svn.wordpress.org/trunk@29404 1a063a9b-81f0-0310-95a4-ce76da25c4cd