This list is not correct.
For example, Player.LocaleId will show:
en-us and not en
pt-br and not pt.
At the same time, there is a complete list of locations in https://locale.roblox.com/v1/locales.
However, this list is unformatted, and also shows the locations with an underscore, for example en_us and not en-us as it comes from Player.LocaleId.
There is a distinction between locale and language – Player.LocaleId will give you a locale, which is comprised of a language (“en”) and a region (“us”). Language codes just comprise of the language (“en”). However, you can use either in localization tables. We’ll work to make the documentation more clear!