Earlier quoted context omitted.
I live in Vietnam and have the same problem I think. My city has diacritics in its name, but the query https://nominatim.openstreetmap.org/search?city=B%E1%BA%AFc-... works fine. Unlike the parent post, it does not go to undefined, it just returns 404 when clicking on the map. Some 404 examples: https://meet.hn/city/vn-B%E1%BA%AFc-Ninh https://meet.hn/city/vn-H%C3%A0-N%E1%BB%99i Working examples in Vietnam https://me…
https://www.openstreetmap.org/relation/1973756 has `name=Thành phố Hồ Chí Minh` and `name:en= Ho Chi Minh City`
I was going to say Da Nang and Ho Chi Minh City have gotten english names indirectly due to their popularity or relevance in Vietnam, but the main capital of Vietnam, Hà Nội, has the same problem as Bắc Ninh where its English name is the same as the Vietnamese name.
I guess there's no logical reason behind this, as this sort of data is likely crowd sourced.