Luna's vintage language · 6:45pm Aug 31st, 2014
Not long ago I noticed the fact that Princess Luna speaks something referring Middle English (maybe Old English, I'm not a linguist), but she speaks ordinarily in S4E1, S4E2 and S1E2, when paying a visit to Discord or turning into Nightmare Moon and back. So, to my knowledge, "Luna Eclipsed" is the only episode where the little sister's English is a bit archaic. As she herself explains, "it is tradition to speak using the royal we, and to use this much volume when addressing our subjects."
This suggests that:
- a thousand years ago everyday Ponish sounded just like that English you are used to hear nowadays;
- its elder form exists, that was assumptive common early in the history, but later was restricted to the sacral, ceremonial sphere;
- according to some researches, the royal "we" means that each "diarch" represents not individual oneself, but a part of the whole "diarchy".
In other words, "We order" in Luna's mouth will mean "Precious I and my dearest sistress Tia..." Also, the royal "we" apparently is not used by sisters in private. What remains unclear to me is the style they addressed to Discord (S4E2). Any "we", any "thou" or "-est", when isn't it better to let ritual words emphasize your might in the face of the enemy? Explanation candidates are: a) he's not their subject, discard the protocol (but see above); b) they reckoned him to be almost dead, nobody would know what they said; c) they were together before, so this private style of conversation is a shadow of their bygone friendship.
Here it ends, being nothing more than summing some facts and thoughts. Hopefully it's correct 
and useful.
I think they consider him more or less equal socially. And the forms you have mentioned are mostly used while talking to persons from lower social groups.
5048958
Must be that. Funny, when I look at this post five years later, its middle part looks like it already contains an answer to the end part. A ceremonial language.
5048958
But still, they are on a battlefield. Could use the "high speech", so to say. Maybe it's the option "c", still. Too late to think clearly, reading a story about Celestia trapped on Earth in the form of an ordinary horse, took me beyond a midnight.
First scenarists throw in a pinch of ancient language to emphasize the millenial gap in Luna's life (and don't throw it in another scene), and then we have to rack our brains to explain it.
I agree, my idea is hardly a sole explanation and purpose of speaking like this. In the same time after I have learned a bit of linguistic I find it hard to believe that ponies speak almost the same language after one thousand years, the same as Luna's. Ponies should put extraordinary efforts to keep their language the same, or they are extraordinary conservative. Of course we can just say that they are cartoon ponies who speak the same language generation after generation, pun intended.
5049176
Indeed.
They may be conservative to some degree, it also may be normal to a world where magic exists. Maybe ancient villain events add to it. I've read a hypothesis that the 1000 years from the legent are actually 1000 moon years, which are shorter) And personally, I think the 1000 years may be not a precise number — it's folklore, after all, plus Celestia behind it, willing to undo the tragedy asap.