Japan Challenge is a new mode introduced in Wangan Midnight Maximum Tune 4 (Wangan Midnight Maximum Tune 5 in the U.S. release). As the name suggests, non-Japanese players got the chance to defeat all players within all the Japanese provinces/regions.
Do be forewarned that unlike usual Ghost Battle, Japan Challenge does not allow you to select course after you picked your opponent, meaning that if there is a course that you don't excel at, you may have hard time defeating your Japanese opponent.
Once you complete all 50 Japanese provinces/region, you rewarded JC sticker
This mode has been selectable through Online Ghost Battle, except if you has registered car, you cannot play it until have 20 trophies to unlock this mode. WMMT5DX+ is last game to feature Japan Challenge as this mode has been removed in Wangan Midnight Maximum Tune 6 and replaced with Multiple Ghost Battle