Japanese publisher GameOn (Pmang portal) announced the Japanese server will go live on September 23rd. The game client is available for download (early character creation will be available today Japan local time through the 21st).
PC based MMORPG’s (not named Final Fantasy) are a rarity in Japan given that consoles have historically been where most of that action has occurred. Japanese PC players tend to be fairly loyal to titles that are able to carve out a profitable niche though (which is why TERA Japan has a stable following there). RPG type games have always as a loyal following so I kind of expect the heavy story aspects of Lost Ark, to do fairly well from a first play through perspective.
The end game aspects has changed since I last played the Korean version and I have no desire to revisit the game as I prefer to just re-experience the changes fresh once the western version is released (so I have no intention of playing the JP version either). It is expected that Amazon Games will be the publisher for NA/EU.