For Australia / New Zealand we allow both answers for multiple reasons of debate including which is more southerly and whether or not English is a dejure language.
Chinese is a bad example... Languages of China are very different and it's in fact wrong to consider them dialects of Han Chinese. "Chinese dialects" is a term used by the CCP to enforce their standard language policy.
Serbo-Croatian isn't a "language" either, following your logic. However, it's undeniable that Serbian, Croatian etc. are just dialects of one language. The same applies for Hindustani and its components (Hindi and Urdu). The difference is political and religious.
Not as of this featured quiz. As per the quote from the Constitution: "The state language of Tajikistan shall be Tajik. Russian shall be the language of international communication." The state language of Tajikistan, which I understand to be synonymous with official national language, is Tajik, and only Tajik.
https://en.wikipedia.org/wiki/List_of_countries_by_southernmost_point
Also what does Fiji have to do with French? 🤔