OK, so we've seen in the show that every single species seen so far, not just in equestria, but other lands as well, speaks what we would refer to as English.

but what if it isn't quite so clear cut as that.

what if instead of there being lots of different languages, all the different countries and races have their own dialects which use what we would refer to as English as the root language, but each with their own twist on it. maybe it can be explained through the magic of harmony creating some baseline level of mutual understanding or something, then over time certain lands and societies have developed their own little take on it.

American-English is Equish, the dialect of equestria, but also a sort of universal language, maybe also used mediatorially in instances where other dialects have deviated enough from the root.

perhaps some places use a dialect which uses english words taken from only one root family, like germanic as with the constructed dialect called Anglish. perhaps as a dialect used among the lower class members of society in some far flung land

maybe places like zebrica or saddle arabia would use something akin to Krio (used in Sierra Leone), which is very heavily based on English, but also takes cues and some words from local african languages

personally, as someone who lives in England, I like to think Trottinghamshire would have examples of some of the more distinct accents, with more archaic words which have fallen out of use here, being used in day to day lives in those lands.

on a more humorous note, this has the potential to lead to all sorts of hilarity, when a word taken as one way in a given dialect has a totally different meaning in another

One note of mine is that Starswirl speaks modern pony, but wrote his journal in old ponish. Luna also speaks modern pony. I believe that these two languages were spoken simultaneously, and that Old Ponish is like Latin, only used for formal texts, documents, and academic writings. It would explain why all the pillars speak pony, though they've been gone since things were written in Old Ponish.

6635272 perhaps, it would make sense for magical documents to be written in a more formal language, especially if said language helps to getter convey certain concepts

6635686
I'm a historian. I understand languages, and since mlp liked to base its languages of ones like Latin and Old Norse, one can take educated guesses.

They call their language ponish in the show. It's also not out of the realm of possibility if they use translation spells, and due to the shape of their mouths, speaking any human language would be difficult to flat out impossible. Twilight did grow up in politics, so she has to know such a spell that she can cast on herself or her friends when needed. That's probably why we hear English.