By initially scanning the `wp-content/languages` directory and loading available MO files just-in-time, plugins and themes do not need to manually load text domains anymore.
Props swissspidy, ocean90.
Fixes#34114
git-svn-id: https://develop.svn.wordpress.org/trunk@37415 602fd350-edb4-49c9-b593-d223f7449a82
