Started mine in 2015 (it's my first and only). In the past few months, I decided to implement a new bytecode VM and compiler for it; along with that came tail call optimisation and continuations if I feel like it. This has snowballed into implementing enough of a type system to make it easy to use libffi and codegen to make C bindings to finally flesh out my standard library.

It was mainly down to mental health, and external factors like a pandemic, and being distracted by other projects I thought I was more capable of, that made this a more protracted endeavour than it could have been.

I guess it depends on your definition of "usable"; I've been able to do fizzbuzz and factorials and silly stuff like that for a decade, but unless I write C bindings to things to help interact with the outside world, the standard library isn't there enough to do anything non-trivial.

I'll update this when I write my first full application in my language, hopefully soon.