tl;dr In many cases, looking at a map with lang=<local lang> shows less labels (and more fallbacks, mostly english) than looking at the same map without the i18n feature.
Localized map with lang=zh: https://maps.wikimedia.org/?s=osm-intl-i18n&lang=zh#13/39.9122/116.3925
Original map of the same area: https://maps.wikimedia.org/#13/39.9122/116.3925
The guideline from OSM seem to be to provide name=<local name> AND name:<local lang code>=<local name> but the latter is not always provided so we end up falling back on many imperfect options instead of showing the local name.
Since we don't have information about the language of the name: attribute I don't know how we can solve this. Maybe we can just accept that the data is not good enough but it feels like a regression when looking at a map with lang=<local lang>