"How are they even talking to each other?"
Picture this: a planeswalker arrives at a plane she has never visited before. She doesn't know much about it beyond rumors gleaned from others who've been there.
Someone is nearby: a young woman who seems harmless enough. The planeswalker needs directions, so she starts up a conversation. And it works.
You've just seen the universal translator in action.
Translation and interpretation have been around for millennia; peace treaties in multiple languages are at least 3000 years old, and the famous Rosetta Stone has inscriptions in two languages and three writing systems. Not surprisingly, fiction must also account for differences in language. Sometimes a work revolves around those differences (as in Ted Chiang's "Story of Your Life," later filmed as Arrival). Other fictional universes extrapolate real-world trends, such as the protocol droids of Star Wars.
The other major approach is to "patch" differences more-or-less automatically, which is often seen when language barriers would get in the way of telling a story. The Common language in Dungeons & Dragons helps keep routine interactions flowing and things like ancient inscriptions mysterious. Star Trek's Universal Translators (capitalized as an in-universe official name) work much the same way, though sometimes translations don't work or take time to help tell the desired story.
Magic: The Gathering exhibits a high degree of universal translation, which seems to ride along with the spark that lets planeswalkers travel between worlds. Interplanar organizations like the Gatewatch simply couldn't function without it, and a story like "Chandra's Origin: Fire Logic" would have ended far differently if Chandra Nalaar hadn't understood the Regathan monk's words of welcome.
But Magic's universal translator has its limitations, and unlike the thoroughly explained Babel fish of The Hitchhiker's Guide to the Galaxy, much remains mysterious about it. (This old Doug Beyer piece is one of the best representations of Magic's universal translator mechanics that I can find, and it's not particularly helpful.)
For one thing, Magic's universal translator doesn't cover idiomatic expressions or cultural knowledge.
One of the most notable recent examples of cultural knowledge influencing use of language comes from the Amonkhet story "Trust." Gideon's awkward hello gets met with a perplexed "What are you wearing?" from their aven greeter, requiring Jace to go rummaging in the aven's mind for the right knowledge to fix the situation. Even then, things don't exactly go smoothly:
"Let me handle this," Jace whispered in [Gideon's] mind, while he stepped forward to address the aven. "Trust me," he said, "clothes like this are the height of fashion in ..." He frowned. "... In the district of Sef?"
As a rule, Gideon wished Jace wouldn't go prying into other people's heads. In this circumstance, though, it was a gift, allowing him to say exactly what the aven expected him to say.
"What were you doing in the desert?" the aven said. "And what did you do to the Hekma?"
Jace turned and looked at the iridescent barrier. "Really? You haven't learned this technique yet ... vizier of the ... Hekma Guard that you are? Well, of course, that's why I'm here in the ... Nitin district, to teach you. With Kefnet. Of course."
Soon after, the Gatewatch reveal their ignorance when they don't know the ritual lines "May his return come quickly / And may we be found worthy" after any mention of Nicol Bolas, God-Pharaoh, arousing the suspicions of Temmet, Vizier of Naktamun.
And when Temmet brings the Gatewatch before his plane's white-aligned God, Oketra the True, there's a reminder of another limitation of Magic's universal translator: names. As Chandra asks Gideon later:
She pulled away and looked up at him with a grin. "So—what did she call you?"
"Kytheon," he said. "Kytheon Iora." The name felt unfamiliar in his own mouth. "That ... was my name. On Theros. A long time ago."
"Kytheon, Gideon. Not too far off."
"No. People on Bant heard it wrong or couldn't say it right and it just sort of stuck. Gideon's my name now."
And yet Magic's universal translator is remarkably resilient in other ways. Even though Jace has no recollection of his own name during "Jace, Alone," he can still read the name of Vraska's ship, The Belligerent, which must be written in either an Ixalani or (less likely) a Ravnican script, and Jace can communicate with her crew.
What new secrets of Magic's universal translator might the Rivals of Ixalan story reveal? I'm excited to find out!