Comment by CapsAdmin
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://meet.hn/city/vn-Da-Nang https://meet.hn/city/vn-Ho-Chi-Minh-City
(technically the last examples should be Đà Nẵng and Thành phố Hồ Chí Minh, but I guess because they are internationally known cities, they use the English names?)
https://www.openstreetmap.org/relation/1973756 has `name=Thành phố Hồ Chí Minh` and `name:en= Ho Chi Minh City`