A visitor from Auckland lands on your site. Their browser sends this along with the request:
Accept-Language: en-NZ
You've got two sets of English text, en-US and en-GB. Neither of them matches en-NZ, so a simple lookup comes back empty and your app falls back to its default. In a lot of apps that default is American English, so a New Zealander gets "color", "organize" and dates written month first.
They asked for English, you have English, and you still gave them the wrong one. To fix that, you need to know what a locale is and how to pick the closest one you've got.