@Ebalosus  
Both of those are loanwords: they both come from “Ceasar.”
 
But since the Crytal Empire speaks fully modern Equuish (despite 1000 year gap) they wouldn’t use foreign words. And the words would be derived from “Celestia,” I think.

What about “kaiser” or “tsar”? They basically mean the same thing as emperor.

I still go by the they have more than one title and choose to go by “Princess (of X)”

@RaineV1  
I guess you have a point. The Eastern Roman/Byzantine Empire was at several points reduced to just Constantinople and the surrounding countryside and constantly got wrecked by both Islamic and Catholic kingdoms, and yet it still retained its Empire title up until the Ottomans put them out of their misery.  
And then there’s empires like the Holy Roman and Islamic Umayyad Empires, which in their later histories were more like a collection of scattered kingdoms headed by powerful nobles and princes that just paid lip service to the powerless emperor/caliph while they did their own thing and squabbled with one another.

@Splinter17  
In fairness, we don’t know about its history. Before Sombra messed things up and made its capital vanish for a thousand years, the Crystal Empire could have ruled other places too, and possibly threatened Equestria; thus being why the Princesses attacked him.

I would believe that once the empire was the most powerful nation on the continent,a glorious imperium of civilized crystal ponies who’s advance crystalic based magic made up for their absence of unicorns,and under their benevolent Queen-Empress the North and the lands that would one day become Equestria Existed in a culturally and economic based unity with the Empire.But when Sombra took over somehow,he reformed the empire to his cruel likings,he suppresses their once intellectual culture in favor for his militarization and crystalic obsession.After being sealed away for a thousand years ,the recently freed imperal citizens have found a new dominant nation,with no knowledge of the new world or military to speak of,the citizens had no choice but to submit as a vessel state to the might of the Equestrian Principality.Fortunately the relationship is mutual,not only has Celestia implaced a ruler of which many of the citizenry believe to be related to their old Empress but the two civilizations have agreed to open trading of ideas and knowledge with both crystal and equestrian ponies very curious of each other and they have even been able to keep their old name despite only holding sovereign over their capital and the surrounding frozen Wasteland of the once fertile lands of the imperial high province.

Queen Victoria was Empress of India, but that title only applied to her subjects on the subcontinent–she was just Queen to everyone else. India’s local ruler was the Viceroy.
 
With its annexation, Celestia became Empress of the Crystal Empire, although she is still just Princess of Equestria. Princess Cadance is the Vicereine of the Crystal Empire, ruling it on behalf of her feudal sovereign Celestia.

@VladimirMacHolzraum  
Empires work by conquering other cultures, maybe they just say the conquered the Wingedgo’s, help a triumph and then went home and had a cup of tea.

@HumanaeVitae  
They control the entire northern part of the continent. But since it’s uninhabited and no one else wanted it, they conquered it simply by scribbling on a map.

Celestia should also be a queen but that doesn’t sell as many toys.

She can call herself whatever she wants.

“Crystal City-State” is more like it, but I guess it doesn’t sell as many toys.

If they are Emperor’s where else has the Crystal Empire conquered?

Considering the Empire appears to consist of about 50 houses and one big building, I think we can safely say no.