I have literally NEVER been able to play a game with more than 5 people with any level of reliability. If I lived in a dorm-room type scenario, maybe. Or, if I can get a group to rent out my local gaming bar, maybe. Other than that - pretty much NEVER going to happen. So, the likelihood of the Japan player getting checked by another player is slim to none unless we put limitations around what countries players can and can't choose (which isn't very much fun).
Yeah, 5+ player games are hard to organize if you want the same players and people you might reasonably know (even if just gaming acquaintances).
But "hard to organize" or being hard to do with reliability doesn't mean it couldn't be done, it just means it's really tough. Besides, 5 players to me sounds like more than enough.
Though with 5 players, you really need to pick your theater of war properly. Center it around Europe, or around eastern Asia, but expecting to do both might be a bit much.
A game with Germany, France, the UK, Russia, and Italy could be a lot of fun, just as a game with Japan, China, the United States, Russia, and perhaps something like Australia could be fun.
Not balanced, perhaps, but at least semi-localized so that players are likely to be able to interact with other players before the end-game.