Given how most of the Equestrian cities are puns off of American cities, it’s written by Hasbro, they have American accents, (Fleur sounds French, Pipsqueak is British, which makes everyone just plain old American,) and the entirety of the Apple family, (plus Rarity, who has a ‘Southern Belle’ manner of speaking,) it’d be fair to say that EQG would probably take place in a really trippy version of the United States. If I had to hazard a guess, I’d say somewhere in Wyoming or one of the surrounding states, considering how little attention the town seems to get.  
On the other hand, a good chunk of the main cast voice actors are Canadian, so it could be some odd prairie town in Southern Canada too.  
I’m probably thinking too far into this aren’t I? Ah well, not like it matters much. In all honesty, I’m just chuckling on how Sonata is Mexican solely based on her love of tacos. I guess that makes me Italian.