Schools in my world of Beckyland can learn just about any widely spoken language as an optional class, though many choose Korean, Mandarin, sometimes Cantonese, or a Romance language.
Mandatory classes are for Japanese and English, which are incorporated into their studies, and they also speak their native creole of Beckynese.
At Focus, Commonthroat is the most widely spoken language. It is the descendent of the administrative language used by the clergy of the Bright Way during the Age of Decadence, when the clergy ruled the entire system as a cyberpunk-esque megacorp. When the secular governments of the inner planets that would become the Allied Worlds (AW) re-asserted themselves after the War of Dissolution, they chose the already prestigious Commonthroat as a standard language.
As the AW grew from a mere treaty of mutual defense to an ever more economically integrated union, other languages were displaced by Commonthroat. At the time of First Contact there are only three “dialects with an army and navy” as it were. The holy world of Hearthside never joined the AW despite being the innermost planet, so they retain a unique Hearthsider language. The Partisans living in Focus’s Kuiper belt, who are the principle reason the inner planets formed the AW in the first place, speak a language called Outlander. The planet Moonlitter (really its many moons) also speak a different dialect of Outlander. Partisan Territory (PT) and Moonlitter are in a relationship analogous to Taiwan and the PRC or North and South Korea.
Outlander is the most vigorous language that isn’t Commonthroat, ironically thanks to the hyper-nationalist policies of the Partisan government which prevent the encroachment of AW popular media. Pups on both Hearthside and Moonlitter grow up consuming said AW media, and thus have a decent grasp of Commonthroat. Commonthroat is universally taught as a second language everywhere that isn’t PT.
Predating all of these by a hundred millennia is Primordial, the yinrih’s written-only language that evolved directly out of a scent-marking behavior. It fills the role of a sacred language in the Bright Way, and is taught in seminaries to aspiring hearthkeepers. Because yinrih evolved writing directly rather than inventing it, they have a written history that extends back to the dawn of sapience.
Meanwhile, the sociolinguistic situation on Earth at the time of First Contact reflects the conditions of today. English reigns as the de facto standard, with other languages like Spanish and Mandarin being regionally important.
Yinrih and humans cannot directly produce one-another’s speech sounds. That doesn’t stop Terraboos obsessed with human culture from trying though. Ideally, both parties in a conversation speak their own mother tongue (or native throat in the yinrih’s case) while the listener passively translates. If one party is monolingual, the bilingual party must use a speech synthesizer to reproduce the other species’ language.