The official UN languages, in the order they were added, are English, French, Spanish, Russian, Chinese and Arabic. The decision to include (or exclude) a language was made based largely on the size of the population who speaks the language natively as well as the geopolitical importance of the countries where the language is spoken. There are plenty of people who think this is an arbitrary, unfair, western-centric system, and that’s probably true. German and Japanese aren’t included, and some say that’s exclusively because they lost WWII.
However, it’s clear that these languages represent seriously large swathes of native speakers, and some pretty powerful countries. Mandarin Chinese is the most widely spoken language in the world, followed by Spanish and English (my native language). In the number four spot, though, is Hindi – notably absent from the UN list, like another South Asian language, Bengali, with is in 7th place. Portuguese is in the number 6 spot, although I admit I never associate Portuguese with being a particularly widely spoken language. Arabic is 5th, Russian is 8th and German (although not a UN language, one I speak and am constantly improving) is 11th. The least widely-spoken of my foreign languages is French. The irony there is that from a geo-political standpoint, French is quite important, and most international organizations use French and English as their primary languages.
In total, once I’m at native fluency in all my target languages, I’ll be able to communicate with 35 percent of the globe’s population in their native language.
I’m also working on German, because I have German heritage, I speak German quite well already, I’ve lived in Switzerland and even if it’s not a UN language, German is an economically and politically important language.