On my 7" tablet if I open google maps for an area and zoom in when on wifi, I find this bit of the map stays in memory/cache even when off wifi. I can't do all the clever google map search type stuff without wifi, but I can use the map. My GPS locates on it as well.
Yes, as long as that stays cached it will be available. The problem is keeping it in cache. Generally you'll have to keep the app open since if you background it, the system can reclaim that space.
Google Maps has a specific download feature to download maps offline. If you try to use that for Japan, it'll say it's not available.