CSO2 Japan (日本語)
Because CSO2 Japan uses the same registration process as CSO Japan, you could follow the registration tutorial here.
Make sure the game client along with the z01 and z02 files from the Download Page are downloaded to avoid any errors during or after installation!
This version is region-locked to Japan only, therefore players residing outside Japan are required to download a Virtual Private Network client to bypass it. Free ones are available such as SoftEther VPN that allow one to change the IP address.
Install the VPN, launch the VPN and change the IP address before re-accessing the site.
Launch the game through the home page (click on the red LOGIN button, and avoid the Facebook/ Google+/ Twitter login!). Do NOT run the client from the Desktop because it would require an additional security step!