I think this UI issue shows a problem in thinking about what the actual problem is that they're trying to solve.
This bug is about contact cards, and so the relationship is necessarily between you and the contact. So, it's not like you need to translate 表妹 into English, or differentiate it from 表姐 in English, as it's something you have chosen to describe the relationship. If you just think of them as "cousin" or whatever you use in your language, that's fine, just use that label. But, once you've assigned e.g. 表妹 to a contact, you don't need to translate that to English if you change your locale... because you already know what it means, because you chose it in the first place!
Admittedly, there's an issue when you receive a contact from someone else, but then you will probably be changing that field anyway. Unless we are family, you sending me a contact for your aunt is unlikely to also be my aunt, and if it is, well I probably also share that native language with you. I'm sure there are edge cases, like siblings who were separated when very young and the parents moved to different countries, or adopted, or found via DNA testing or whatever. But even in those few contexts, learning about the word or having the person themselves explaining the relationship to you, probably helps overall understanding of the relationship.
In any case, a lot of the Chinese words for the various familial relationships are quite amorphous. I remember being very surprised when a Chinese friend posted some photos on Wechat of her 孙子, literally "son's son". My initial reaction was "She can't be old enough to have a grandson!" then "Wait? She has a son? I thought she was single..." Turns out, she was referring to her brother's son. Lots of people use 哥, 弟弟, 妹妹, 姐姐 (literally older brother, younger brother, younger sister, older sister) to mean their friendship group, although this is probably a result of the impact of the one child policy. 阿姨 and 叔叔 (aunt and uncle) are just respectful terms for people of similar age to your parents, etc. There's literally no point trying to translate these names into their literal English meaning, because it will just further confuse people.
So, then the big problem becomes where these labels come from. Is it a dropdown box where only the options available in your current language are presented? How do you then choose one that's available in a different language if that's what you want to use? Presumably these aren't translated too literally into other languages, as e.g. an English speaker isn't going to want to choose one of 4 options for "cousin", and hopefully would be immediately suspicious of why their phone wants that much information about the nature of their cousin-ness, when it's customary to just use a single word. How then would that get translated back into Chinese? It would be embarrassing at family events if someone saw that the male cousin on my mum's side was labelled as female cousin on my dad's side.
I think it'd be much easier if you could just type what you wanted into this relationship field, maybe with a drop-down or auto-complete for likely options in your language.