Base is not a language, but it provides translation. Base localization, when set as the development language, also helps differentiate "development" from production. It defines a language that is usable for developers and won't be visible to users. This lets the developers work with actual translations that provide context and are immutable, while the translators can update and change the wording depending on "a better use of the language", marketing, or business correction. With xcstrings, we haven't found a way to differentiate "developers' English" from "production English". This is a requirement for our development and translation teams. I don't think there is any way to have Xcode build the Base.lproj folder; this is why we picked "und," as it represents an undefined language.