Hi, author of Jacqueline here. I read HN almost daily so it caught me off guard to see my stuff here.
It's been a long time since I did this (2019). It was a prototype just to see if a standard PC boot loader could hand-off into something that's not C (or Rust). And yes you can, as long as the programming language has a way to control how symbol names are exported, and then to link the object code with the rest of the boot loader.
You won't have a runtime unless you implement one, so for most languages there is no stdlib, no exception handling, no garbage collector... But it is fun anyway. As I said, this was a prototype and once it could say Hello World I considered it complete.
Happy to see it here though, and I'll be happy to answer any questions about what I remember, or what is like to write code in Pascal, or OS development or i386 in general.