We did this in 2010 and used Japanese characters in the URLs. Do you have any recommendations besides hreflang that might help us using GeneratePress? Since the site is going to be used by multi-lingual people, I don’t believe we will install the Japanese version of WordPress.
I don’t think there’s anything specific you’ll need to do – at least on the GeneratePress side of things. As long as you can make it work using WordPress, GeneratePress should be fine 🙂