This is something I'm super-passionate about. Back when Ornn was released, we talked a lot about the "ancient Freljordian" language that was created for 'The Path to Hearth-Home' teaser song, and I'm always trying to find ways to do more of that kind of thing.
It's probably safe to assume that a lot of nations across Runeterra speak a common tongue, or that they at least understand one another. It's important not to assume anything... but that would lead me to question, where did that common language come from? It has interesting implications for the lore. :-)
In a real-world sense, speaking from 10+ years in publishing and novels, if a story features only characters speaking their native language but it is intended for an audience who don't speak that language, you just do it in the language of the audience. I'm thinking of the TV show 'Versailles', which is mostly French characters, almost all speaking French... but the show is in English. That's just suspension of disbelief, for ease of consumption.
'Vikings' is a great example of messing with that idea for the purposes of dramatic storytelling. When you have only Viking characters speaking their language, it is presented in (Nordic-accented!) modern English. The first time we see English monks, they speak Old English and Latin, but then transition into modern English as we get to know a couple of characters in particular. HOWEVER, when a scene is from the POV of the Vikings and other characters are speaking Old English (and vice versa) things start to get interesting!
The TL;DR is, basically, you can use language, and language barriers, to serve the needs of your story. Otherwise, just find a way for there to be a common tongue, and lean into that.
Great question, though!
