In CLDR, the Chinese language for China is listed as zh-hans and zh-hant, and not as zh. When ULS tries to find whether a link to zh exists, this fails because our links are only zh and not zh-hans or zh-hant.
This also happens with ug-arab and kk-arab, which we have as ug and kk.
zh may end up in the list despite this thanks to the recently-added extra common languages (T135366), but we must not count on it, and instead we must cover it with the CLDR territory info by converting zh-hans and zh-hant to zh.