In Demon: the Fallen, each Fallen has three names:

The mortal host's name, the fallen's Celestial name, and the fallen's True name.

Of the three, there have been slight descrepancies between the way Celestial names should function. According to canonical rules, the Celestial name is the name the Fallen used to refer to one another during the Fiat Lux. The names, spoken in Celestial tongue, permitted the Fallen to converse despite far distances and varying planes of existence.

In the modern day, though the ability to communicate through the invocation of the Celestial name still exists, there has been questions on exactly how a Celestial name can be said if it is supposedly Celestial Tongue in origin. Though the rules suggest to use names from various religious, mythological and fictional sources the question for gamers still arises, "Where does the distinction lie when a name is merely a word invoked or a word said?" Especially when some Celestial names, like Azmodeus, Azrael and the like are quite common in modern day fiction.
Here shall be my attempt at resolving this issue.

Multi-faceted Reality (its a WoD setting that a ST made up to explain the inconsistency with true names. interested?)
It is said that in the beginning, reality existed in many planes. There were nations of man, just as there were dinosaurs, just as there were only Adam and Eve.

From this multi-faceted existence, reality was forced to exist merely in one single plane of existence.

With this in mind, one must then realise that just as the True name of the fallen are composed of numerous elements, sounds and harmonic resonances, so too are the Celestial names just as complex and numerous. But where as the True name's complexity lies in its numerous facets that create a single name, the Celestial name is numerously faceted in its representations.

To put it simple: A True name is composed of many frequencies. Where as a Celestial name has many frequencies but only one meaning in the end.

Clear so far?

Explanation by Example
It should be clearer if I explain this way:
A Defiler, back during Fiat Lux, was called Bringer of Salty Tears. His True Name consisted of fifteen varying ways water can splash against flesh. His Celestial name at the time was simply: Mit Hephis.
But after the Time of Atrocities and the other ages, his names began to include Mythepis, Maithepis, Mythepys and Meithyphis. This was due to the many tribes of man whose languages began to have different culutural, habitual as well as social intonations and forms of writing/speech.

After escaping the Abyss, the Defiler discovers that humanity has remembered his existence somehow. Books of both myth and lore speak of his deeds (and sins) and yet have added two new Celestial names to the demon (due to varying locational and cultural language): Mi Thypis and Meethep Hsis.

Now, how does this work in system?

Invocations and Rules
Using the earlier example, though the Defiler's True Name has never changed, the Celestial names that can be used to invoke the demon are now manifold. But, only one of the many true names resonates the most with the Defiler, and that would be his original Celestial Name.

System wise, it would be approached this way:

• Invocations using his True Name which consisted of fifteen varying ways water can splash against flesh, would follow the same rules as in the book.
• Invocations using either Bringer of Salty Tears, which was his original title, or Mit Hephis, his original Celestial name during Fiat Lux, would also follow the book's rules.
• Invocation using any of the following:Mythepis, Maithepis, Mythepys, Meithyphis, Mi Thypis and Meethep Hsis would be at +1 difficulty because the name is less resonant to the Demon being invoked.
• Add an additional +3 difficulty if the invocation was non-intentional (such as a curious teen reading a book on demons, or if the name has become a household term or common name in pop culture, like Azrael or Lucifer).

In the end, you maintain the idea that a Demon can be accidentally summoned or called, but at the same time, maintain the importance of the original Celestial name, as well as avoid the question on how a Celestial name should sound since it was in the original language which no longer exists today.
Obviously, this system is more flexible, permitting a demon to choose exactly what to share with his Thralls.
Obviously, a mortal name cannot be used for Invocations