Figured out the dropdown from an old YouTube Masthead Close Button download I had earlier.
"gwdSpecific": {
"type": "dropdown",
"items": [ "af", "am", "ar", "bg", "bn", "ca", "cs", "da", "de", "el", "en", "es", "et", "eu", "fa", "fi", "fil","fr", "gl", "gsw", "gu", "hi", "hr", "hu", "id", "is", "it", "iw", "ja", "kn", "ko", "lt", "lv", "ml", "mr", "ms", "nl", "no", "pl", "pt", "ro", "ru", "sk", "sl", "sr", "sv", "sw", "ta", "te", "th", "tr", "uk", "ur", "vi", "zh", "zh-cn", "zu"]
},
This is not in the documentation. It's magic! :)
Also, these don't seem to be documented. It's easy enough to guess their functionality, but would be useful to know they existed without having to dissect custom components to find them.