@FanOfMostEverything  
This.  
This explains everything.

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@Yet_One_More_Idiot  
sry for the late reply
 
It’s kinda like this: The Latin word “princeps”, from which “prince” obviously comes, was actually a more-or-less general term for any head of state or governor - meaning, something closer to a Prince that is leader of his principality or princedom. The word, literally, means something like “first seizer”, but was/is understood as “the first” or “the chief”. Compare with German Fürst, Norwegian and Danish fyrste, Icelandic fursti, Dutch vorst, etc. – all comparable and cognate to the English word “first”, but referring to the archaic meaning of “prince”, which in English is muddled with the more modern meaning.
 
Now, I look up Old English translations for princeps in the Bosworth-Toller dictionary and it returns me several possible equivalents, including brego, dryhten, ealdor (and derived), fengel, etc. And if I look for “prince” itself, I get even more results, some more or less relevant than others.
 
But, the child of a king – I keep looking and keep getting only æðeling/æþeling, “nobleling, noble one; etheling”. Take for example the translated text in the second part of this entry: in the first paragraph, Aethelstan King is mentioned alongside his half-brother Edmund, who is called an aetheling (and who would succeed Aethelstan years later); both were sons of the former king of the Anglo-Saxons, Edward the Elder, son of Alfred the Great.
 
Looking up “princess” gives me but a single result, and a hardly even relevant at that.
 
All in all, and back to the show, I guess Celestia and Luna could be the equivalent of principes, since they’re ruling princesses on their own. If they’re daughters of a late Equestrian king, well, the show has yet to explore their background in-depth.

@OverlordScorpion  
I thought that there was actually a word for Prince, just none for Princess? “Royal ones” is basically the same as us referring to “the Royal family” nowadays. :) Although obviously we only use it collectively, not to refer to an individual royal (although we can and do refer to an individual member of royalty as “a royal” like I just did in that very sentence, so I guess that still holds too) :)
 
@monus783  
Well with Discord’s level of chaos magic at his disposal, maybe he can just make everyone understand him without actually having to speak Modern Equestrian, or somesuch? xD

@Yet_One_More_Idiot  
Discord would also have to speak that way, but again he’s chaos incarnate so it wouldn’t apply to him.

@Yet_One_More_Idiot  
That’s right. The king’s family were called just aethelings - “royal ones”

…although I just realised that technically, Twiley is saying, “That was good, Queen”, rather than calling Celestia “Princess”. xD
 
Apparently, there was no standard female word for the female equivalent of a Prince until at least the 17th or 18th centuries in England; before that, a lot of Princesses were simply styled Lady or The Lady.

Ah, proper Old English! I would love to have seen S1 Luna speaking in Old English with subtitles in both Old English and Modern, to show how really behind the times she is (1000 years ago, after all, would be OE, not the Early Modern form she typically speaks with). :)

“I see you’ve been talking with Luna. Don’t worry, we should have her up to Early Modern by Nightmare Night.”