Jython should be fine. I believe it uses the script API.
Nashorn JavaScript appears to become deprecated soon so it’s future is in doubt.
The NGRE is a way to write Rules using PaperUI. Currently it only supports Nashorn JavaScript. See Experimental Next-Gen Rules Engine Documentation 1 of : Introduction. By all appearances other languages will be supported at some point but at this time there will be a good deal of new work necessary to NGRE (and the docs I’m working on, grumble grumble).
In all likelihood someone else (not Oracle) will pick up implementing some sort of JavaScript alternative that works with the Java Script API. But whether that is in time to give OH continuity is not known.