It's highly likely that the original implementation language was assembly. The code is very idiomatic.

Regarding source build, I think reverse engineering it to the point where you can reconstruct the source is possibly legally problematic, so I don't plan to do this, but maybe for certain subsystems like MathPrint (equation display) which was especially fun to RE. I have a PR up for it and it will be live at

https://siraben.github.io/ti84p-re/mathprint