In this manual, i did not find anywhere that is actually romanized as "Joylant", and yes it is ジョイァント as is written, which they had decided to romanize it as Joilant for the American version, but there is no romanization here, so no, it's not Joylant in the Japanese manual. But it is obvious that Joilant is meant to sound like Joy Land.
But on its left, there is 喜びの国 (よろこのくに), which basicaly means "Land of joy", the full phrase can be "The land of joy Joilanto)
Also, no, the case you brought for D and T is wrong, there is a D variant for that, ド is the "do" one, ト is for "to" basicaly, just pay attention to the little extra above that line.