I want my iOS to recognize both British and US spellings as correct, but to prioritize one or the other depending on what the primary language is used (but it is a secondary language).
I found the following solution in particular: How can I change the language for automatic spelling correction? This is something that cannot be reproduced on an iPhone. It still shows one of the spellings as incorrect.
I also found a solution to force British spelling and based on that I tried to implement what I wanted but failed.
I think the issue is which English version your keyboard is set to. Likewise, I couldn't find an “English” or “General English” option for the keyboard language, something I sometimes see on the web that adjusts for both spellings.