The closed beta for the japanese version of Warship Girls (Senkan Shoujo R) is scheduled to start from September 30th 12:00 UTC/GMT+9 and will last for a week to October 7th 11:59 UTC/GMT+9.
If you're native japanese or understand the language you can apply for the Beta via e-mail with the header 「戦艦少女R日本版CBT申請」to SenkanSyoujyoR@moefantasy.com by September 29th 11.59 UTC/GMT+9. Please only apply if you actually understand the language.
The Closed Beta is only for Android devices and all closed beta accounts will be wiped after the testing period however beta testers will be rewarded on the official release, if it's anything like the chinese beta you'll end up being the owner of Goeben
The japanese version is currently expected to be their own seperate servers so it'll only be rewarded on there.
More info can be found on the official Warship Girls japanese twitter here