Nautilus armor uses the American English spelling (armor) rather than the British spelling (armour) when the game language is set to British English. This also affects the Canadian, Australian, and New Zealand English languages. This is inconsistent with all other types of armour, including horse and wolf armour, which are translated correctly. (Sorry if this needs to be reported elsewhere because it’s a translation issue)
Steps to Reproduce:
Set the game language to UK, Canadian, Australian, or New Zealand English
Obtain any type of Nautilus Armour
Not a bug per se, the issue is caused simply because non-default languages don’t yet have translations for brand new content. May be worth to mention it on Crowdin, but likely Mojang staff is pre-emptively aware, and will make the translations when it is relevant.