TinyMCE:
- Bring back loading of /langs/[locale].js and /langs/[locale]_dlg.js from PHP. Prevents errors with missing translation files when custom plugins use requireLangPack() without second argument.
- Back to using ISO 639-1 (two letter) locales as the locale is used as part of the translation file name.
See #24067, fixes #27610

git-svn-id: https://develop.svn.wordpress.org/trunk@27922 602fd350-edb4-49c9-b593-d223f7449a82