Map CKEditor languages to Drupal languages

Summary

We need some way of mapping Drupal languages to CKEditor translations/languages.

Original report

From #2036253-12: Update CKEditor library to 4.2:

Well I'm not sure we want to pick up whatever is offered to us, because we don't want to ship CKEditor translations for languages we're currently not supporting with a Drupal translation, such as currently sr-latn, fr-ca or en-** and probably more to come. So optimally we would be using LanguageManager::getStandardLanguageList.

Either way we need to find ways to match language codes like pt-pt => pt, no => nn, zh-cn => zh-hant, and zh => zh-hans (Let this be a followup, though).