Comment 3 for bug 47280

List of 'preferred languages' is a list of languages you are interested in translating into.

US English is standard for most projects (i.e. at least for Gnome[1] and KDE[2]), so there is usually no need for en_US. Reasons for this are multiple, but apart from consistency, it also allows better translation memory reuse (i.e. translators will be able to easily reuse translations from other programs using American English as their base language).

As for other variants, I know en_CA has definitely some different translations and spellings from en_GB, at least in Gnome. I am not sure about en_NZ, but if there's an actual difference from en_AU, then I am perfectly fine with creating it: provided you actually want to provide translations for en_NZ.

If this is only about display (as compared to functionality) preferences, we might want to figure out a way to provide "aliases" between languages.

[1]http://developer.gnome.org/documents/style-guide/grammar.html
[2]http://l10n.kde.org/docs//styleguide/other-consistency-issues.html