I've completed a Hebrew translation on Transifex.
Is any further action required to have it included in the next version?
Is there some way I can preview it before it's included in the next version?
How can Japanese be added to this list?
Took me a while to figure out, but apparently you need to log in, go to https://www.transifex.net/projects/p/bitcoin/resource/tx/
(note: This is a different link from what paraipan gave, where there is a different option that doesn't work), and click "Add New Translation".