SOLVED!
I read about KIT Scenarist, the sort of standard screenwriting software on linux, in the latest edition of the (German) magazine linuxUSER. The article's author didn't mention that on the KIT Scenarist's website there is also an AppImage that you can install (or rather: run) on linux systems that are not part of the DEB or RPM worlds.
I just happened to find that out by coincidence.
So, I downloaded the AppImage (latest stable version), integrated it in my system (by using the wonderful menulibre!) and ran it:
Alas, it works just fine, with full language integration and all, and I don't even have to install it. Hooray!
Trelby:
The author of above-mentioned article wrote in another article that Trelby was based on Python 2.6/2.7 and that there hadn't been any changes to its code since 2012! So, outdated.
Thing with package requests is that I wouldn't be able to judge if
- KIT Scenarist can be re-packaged for SOLUS, if that would be allowed,
- how I would have to find out if it would be.
Solved my own problem, it seems! 😄
(And, by the way, it's my daughter I need screenwriting software for; her lappy is running SOLUS, too...)