In recent lore there was a guardian who originally live about 13000 years ago and had to learn modern language because their ghost didn't give them the knowledge. Shaxx has a Scottish accent. Saint is Greek or Russian Osiris is possibly British In Y3 there was some lore of fallen learning human language and speaking to the apprentice speaker. Then you have Sjur Eido learning Eliksne while mithrax learns English from her.

Cabal, Hive, and Fallen are able to learn human languages, and usually important ones do, like Kells or Hive gods. We don't tend to learn their languages, although some people do such as Eris and Toland learning Hive Runes. Our Ghost can translate very language for us. Throughout the franchise, he's translate Hive, Fallen and maybe Cabal, as well as rasputin.

I always thought it was translation by ghost but in the beginning of season of the risen Caitl quotes sun tzu's art of war which implies that the cabal are either capable of translation too or that Caitl took the time to learn English (or Chinese at the very least) to read this human text. So maybe they just learned it too.

For the humans, I assume they realized so many languages in one city wouldn’t work so they decided to use whatever you have in settings. I think everyone else took the time to learn English in preparation for meeting us, though I’m not sure why Eramis would care.