@feditips @ButterflyOfFire Mastodon does not define alternative codes, so 3-letter codes are only available for languages that don't have a 3-letter code.
The code lists are in https://github.com/mastodon/mastodon/blob/main/app/helpers/languages_helper.rb
The codes listed in ISO_639_1, ISO_639_3 and ISO_639_1_REGIONAL are available as posting languages, the codes listed in REGIONAL_LOCALE_NAMES are not.