That’s not the use case I imagine. When setting relationships in a Chinese localization this option appears with the correct characters in the dropdown. If that contact is shared with an English locale the attribute is still set but it displays as “cousin”.
People can use iPhones in English and be native speakers of languages that make the distinction; why would you hide that information from them?
Interesting perspective. I define the rules based on the localization but maybe it should be by user. Not really sure how that UX works though.
To solve both that use case and the ones you have in mind, the API could perhaps be extended with a language-aware "normalization" procedure that maps relationship labels in a way that loses information but produces a more canonical label for that language. And you'd use that API only in those cases where it makes sense.
But then that would lead to a contact being displayed slightly differently in different settings, which may in itself be confusing to users.
At some point you just have to give up and call it good enough.