Kia ora, this is not about setting the default language for common name. Pōhutukawa is the common name in English, and is now in NZ English predominantly spelled with the macron, just as it would be spelled in Māori. There are many NZ species for which the common English name is a Māori loanword word; usage has recently changed and these words are now spelled with macrons in almost all reliable sources. So the question is can we replace the macron-less English names with correctly-spelled ones? We’re going through this same issue in Wikipedia at the moment.