It seems that "Ar Ciel" is the planet where the game takes place, though Carlton admits that the previous two Ar Tonelico titles named the same planet "Al Ciel." This was done for a very specific, innately nerdy reason: "After all of the concerns that were expressed to us about changing some of the names from their original Hymnos names (especially for the second game), we have decided to put more emphasis on staying true to the Hymnos names." Things, it seems, have gotten a bit off the rails here.
What in the world are "Hymnos" names? According to an Ar Tonelico wikia page, these are the names originating in the "Hymnos" language of planet Ar Ciel, a "common, spoken language" that "has fallen out of use by the time of the main story's events (circa A.D. 3700)." In so many words, the game's title is somewhat more confusing than previous entries in order to stay more in line with the franchise's deep, deep lore. Beyond Hymnos being a language in the world of Ar Tonelico, it's a language created for the games by franchise director Akira Tsuchiya, derived from English, Japanese, and German. Seriously.
In case you're not versed in the world of Ar Ciel (like we weren't), apparently its people live in "Ar Tonelico" buildings, described as "the type of artificial towers where everybody who survived the global catastrophe of roughly 700 years ago lives." And with that, as Carlton points out, "that only leaves a few more terms in the title that need to be explained."
As noted above, "knell" is "like a chime that signals the passing of someone or something." Carlton contextualizes the word in its most commonly heard use -- a "death knell." Carlton says, "This keeps in line with the musical theme of the previous titles, but makes it a little more somber than a simple melody." Moving back into words that don't actually exist in any real dictionary, "Qoga" is "a Hymnos word that can be roughly translated as 'finale' or 'termination.'"
Piecing all the parts of the puzzle together, the title roughly translates (from "English") into "The Finale of Ar Tonelico: The Death of the Planet." While we appreciate how much less eloquent that name is, we can't help but admire the simple beauty of "The Death of the Planet" as a sub-title. It's certainly what we'll be mentally replacing "Knell of Ar Ciel" with!
"Many people may look at the title Ar Tonelico Qoga: Knell of Ar Ciel and wonder what that jumble of seemingly random letters mean. Well, to help clear things up for all of you, we here at NIS America, Inc. have decided to provide this explanation. There are a few key terms from the game in the title, mainly Ar Tonelico and Ar Ciel. Ar Ciel is the name of the planet where this game series takes place. Now, some of you may be thinking, "What a minute! I've played the other games, and I distinctly remember that the planet was called Al Ciel! What are you guys trying to pull here!?" Well, yes, that may be true, however, after all of the concerns that were expressed to us about changing some of the names from their original Hymnos names (especially for the second game), we have decided to put more emphasis on staying true to the Hymnos names. Therefore, Al Ciel has now become Ar Ciel, whether or not anybody actually complained about this specific Hymnos name before.
Ar Tonelico is the name of the type of artificial towers where everybody who survived the global catastrophe of roughly 700 years ago lives. Now, that only leaves a few more terms in the title that need to be explained, in order to understand the complete meaning of the title. I will explain them in reverse order, to build suspense, and so that some of you don't get ahead of me. Knell, more commonly known as death knell, is like a chime or ringing of bells that signal or recognize the passing of somebody or, sometimes more symbolically, something. This keeps in line with the musical theme of the previous titles, but makes it a little more somber than a simple melody. Now, Qoga is a Hymnos word that can be roughly translated as "finale" or "termination". Now that you know what all of the components mean, you can see that the title can be understood as "The Finale of Ar Tonelico: The Death of the Planet". From here, you can kind of get the picture of what some of the characters in this game are concerned about."