personally, I think the Unown hold the cards. Following the lore, it's hypothosized the language written and possibly spoken evolved from these strange floating character Pokemon. Since the Unown are universal to Johto, the Sevii Islands and probably mainland Kanto, and Sinnoh, I'm betting that they're all on the same page. They may bhave different distinctions, but look at Spanish and French, theres a lot of similar words in their own languagesm because they came from the same base language.
Here's a thought then, since Hoenn uses a Braille language and makes little mention of the Unown, as does Unova, they may actually have evolved a differnet language, That means you only have 2 languages in thwe Pokemon world so far, naybe three. Also, since the Orre region is based off another american location, they're likely speaking something akin to what Unova is.
Now , from there, I question global uinity. Odds are good that if trainers are wandering freely that there's a tight international network, and that wouldn't be hard to create a universal language for out of three known languiages, so you end up with Old School Unownese, or what I call Quertisian in my fic, Unovan Braillian and Hoenn Braillian, and a hybrid language new World language of the three. That's my take.
I've never been a big fan of the real world references, to me they are a metaphor that communicates a stereotypical character, nothing more. Although it's possible that the "America" Surge came from is actually a different-shaped landmass and entirely different than what we think of. Don;t give me that "based off" nonsense either. Lot's of D&D maps are "based" on inverted Earths, that doesn't mean it IS Earth, just that they made a nice canvas to start with. Most Star Trek planets were filtered versionsof planets around Earth, or Earth too, but they weren't Earth.
And also, if we're still in the game universe (as you've already dismissed the anime universe for these purposes) Fortree city is only a short walk from Mauville or Lilycove, two urban centers similar to Sunyshore, which can be reached in a matter of minutes on foot. It's unlikely that unique dialects would develop in such small areas.
In model railroading we have a term: "Selective Compression". It means that the distance between two major scenes, like two cities for example, is a loosely scaled representation of a much larger distance. For example, if I have an actual 20feet of farmland between two major urban centers, that scales down to less than a mile in 1:87th scale. (twenty actual feet is 184feet scaled) That's a rediculaly short gap between two cities, correct? At a minuimum, it should be twenty
miles, not twenty feet. But mlike my model railroad, the gamecards and the gameplay itself is limited It can't put an accurate amount of distance between Mauville and Fortree and Lilycove, so they "model" a "selective compression". In actuality, yes, it's a only s few thousand steps, a short run and a jump. But if you logically unscale that, even assuming Hoenn is an island region like Hawaii, you've got room for minor changes in dialect.