Numbers of speakers and/or numbers of countries in which languages are spoken are probably more important. Japanese, German and Italian are not as widely spoken as the official languages.
Eight, according to Wikipedia. Two of those countries are tiny (Luxembourg and Liechtenstein) and two others are rather small as well (Austria and Switzerland, which has large parts that rather speak French or Italian). In three of the countries, it is only a regional language (France, Italy, Belgium - the German speaking community in Belgium is tiny).
Why though. Hindi is only spoken by one UN member state (India, where most people in power speak English). Furthermore, India isn't a permanent member of the UN security council which was the justification for Mandarin and Russian.
Political distinctions make it hard to add Hindustani. Hindustani has no overarching, formalised standard to draw from and choosing an official script would be contentious.
Given that French is the official language of Diplomacy, that one is understandable. Russian, English and Chinese probably correlate to how the five main members of the UN are USA, UK France, China and Russian, and Spanish and Arabic... probably more prevalent compared to other languages, if I had to guess.
Also, it is still spoken and known throughout Russia, Eastern Europe, the Caucasus, and Central Asia. It is also the most spoken Slavic language with a Cyrillic alphabet, which is why it almost represents many other Eastern European languages
A nice quiz, thanks for sharing.