To get a 6IV Ditto in game u have to search a lot...
And everyone wants a japanese cause due to the Masuda Method that way u can have more chances to hatch a shiny poke...
Just ask any pkmn breeder xD
The main reason why someone wants Japanese Dittos, or any ditto from a country they're not from is because it raises the chance of getting a shiny pokemon through breeding, http://bulbapedia.bulbagarden.net/wiki/Masuda_method
With the ditto though, it's best to check their IVs through an IV calculator. You can grab Dittos through a mirage spot. Here's an IV calculator: http://www.serebii.net/games/iv-calcxy.shtml
I believe the chance to encounter/breed a shiny is something like 1 in 1300. That being said, I've only hatched about 1400 eggs and I've already got 3 shiny pokemon just from that and that was without the masuda method
For starters, there is a slight chance that you can find shinies in ORAS/XY. Unless you have insane luck, it's literally impossible without the shiny charm if you wanna go shiny hunting in wild grass XD