I remember asking "wtf is Qoga?", someone said, "it means 'life' in the ArTone language!!!!!"
WHAT THE HELL GOOD IS THAT TO ANYONE?? I thought that was a terrible name, without a doubt ostracized some people curious about the title and can't make sense of it... it was bad enough "Ar Tonelico" technically means nothing beyond fancy being a video game name.
JRPGs can be pretty bad for these in-game referencial titles. Radiant Historia is a bit of a mouthfull (and just down right confusing when people remember a similar-sounding PS2 title), and again, doesn't really mean anything.
Although actually these two titles do make sense in the context of their respective games.
!!SPOILERS FOR BOTH GAMES BELOW!!
"Qoga" translates to "end, finale, demise, death" from Hymmnos, and is a very direct clue to the fact that the game is not only the finale for the Ar tonelico series, but also to the fact that this is the one that decides the fate of the world of Ar Ciel. To put it simply, anything short from attaining any of the four true endings, and the planet dies together with everything living in it. As for "Ar tonelico", it has meant "The Only Divine Tree" since the first game, making a reference not only to the Ttwers being a symbolical representation of the Yggddrasil (world tree), but also to the fact that they are the only place in which life can continue existing and thrive after the catastrophe that destroyed the surface of Ar Ciel and rendered it uninhabitable. Maybe it have been better to leave it as Ar tonelico III, but still isn't as unwieldy as having translated the entire title literally. Or does something like "The Only Divine Tree Finale: Knell of the Only World" sound better?
As for Radiant Historia, if I recall correctly, the main goal of the game is to create an ideal timeline in which the world doesn't perish in the end due to the desertification. And well, we all know what "Radiant" means, while "Historia" is the Spanish, Italian, Portuguese (and several other romance languages) word for "History", all of which descend from the Greek word ἱστορία (istoria). So we can reduce the name to "Shining History", with the title referring to the ideal timeline that the characters are trying to create in it. Likewise, the place from which the traveling between the timelines and to different points inside them is called "Historia" as well.
Personally, I don't really think that a strange title has actual bearing on the sales of a game. Most people will actually look at the cover, read the summary on the back or seek information online before trying to buy anything.