I was going to reply by suggesting emacs lisp as a candidate language, really making it a bet on how long emacs will be around. Will people (commonly) be using emacs in 50 years? I think people will, though I hesitate to say so. If it turns out that we converge on text as a necessary interface to a computer (at least in some cases), maybe the bet pays off.
But I think your idea that the expression of a program will become fungible or machine-translatable is much more salient. Though if the program itself depends on a whole chain of ancient dependencies and idioms (think a VB UI in front of an Access DB) might run afoul of infinite regress. So, to really future-proof on a long time-horizon, it seems you need to be preoccupied with a lot more than the programming language.
But I think your idea that the expression of a program will become fungible or machine-translatable is much more salient. Though if the program itself depends on a whole chain of ancient dependencies and idioms (think a VB UI in front of an Access DB) might run afoul of infinite regress. So, to really future-proof on a long time-horizon, it seems you need to be preoccupied with a lot more than the programming language.