Mandarin isn't actually a standard, it's that China has a fuck-huge population and a massive ethnostate complex to keep a single language for that fuck-huge population. Arabic is in a similar boat of being widespread solely from a cultural homogenization process, and as such is also quite limited because you're essentially only opening up the Muslim world, which is fairly limited in value to be speaking to from how fucked over those parts of the world have tended to be, and you get very limited breadth because it is in fact the Muslim world.
To my understanding, French isn't used as such anymore because English took over the spot of "language of the hegemon" thanks to Britain winning the Age of Empire and the United States winning World War One, but remains in the diplomacy courses and other such places out of incestuous self-reference and institutional inertia, so it's useful because it was useful so most people who've learned a second language will have had the option of French.
Spanish faces a similar issue to Mandarin and Arabic, in that there's a lot of people who will speak it "on the street", but there's a fairly limited bredth that actually covers because of those people being similar clumps because they are in fact Spanish peoples in origin, though it does share in the prevalence of French as being "locked in" for available education in other languages.
English stands out because you actually have to learn it in some capacity to interact with a variety of practical uses. Programming languages are nigh-exclusively based on English so you have to learn it to understand large instruction libraries, the US and UK remain obscenely disproportionate for quantity and quality of scientific research so academics end up stuck getting some footing in it to readily reference relevant works in their field, the vast majority of the giant multinational conglomerates are based in the US so if you want to directly work with them, you need English available.