This isn't entirely true. People are saying that it can't be in Japan because they assume that the regions are located in the same places are their counter parts.
However if this is true then Ash must travel through a lot of uncharted land to get from one place to another in the anime.
Also geography says each of the in-game maps should have the top the normal North-East-South-West angles which also throws this proposal off.
In-game explanations of regions positions and anime ones tend to indicate that they're in their real world locations. As well as just the concept art maps. And he wouldn't have because he traveled by boat.
Its very far away, yes, but that may be only relavent in the world of Pokemon, they could still use japan, plus, if you were to walk from the top of japan to the bottom i suppose it could be considered "Far away"
-Dee
The top of Japan is Sinnoh and the bottom is Hoenn.
IMO it looks like there is only room for about 2 more regions in Japan. They had to ditch Japan for something new.
This. I don't get why people are so desperate for them to use every last bit of uninteresting landscape in Japan before they can look at the rest of the world. They've covered almost all of it.