> that existing Nix code could be translated into,
It is highly unlikely that this is feasible. I don't know nix in detail but I infer from your post that it is an untyped language. Untyped languages can, in general, not be translated to (statically decidable) typed languages.
> which could then be abandoned.
It could not, unless you convince a majority of the contributors to the package repo to switch to your new language.
Some more complex nix features would not be easy to transpile automatically, but many (i don't have stats sorry) packages are very simple declarations which could be ported automatically.
For example, all language-specific libraries imported from rust/python/etc could very easily be automatically ported, and in my experience, writing declarations for those many dependencies is what takes most time in making a small package (although admittedly i only tried once).
Nix, is a dynamically typed language, i.e. the language gives every value at runtime a type, but there a no compile-time (i.e. static) checks whether any function or operator calls will match at runtime to their parameters. Programs can be translated from dynamically typed-languages to statically typed languages. Either by infering types at translation time or by exhaustively adding lots of cases to every function or operator call depending on the runtime. The first approach can in principle only translate a subset of all programs, the latter does not give you any additional guarantees, because you have encoded an interpreter for the dynamically-type language into the statically typed language.
Whether the first approach could be used on a large codebase like nixpkgs is highly questionable, the second approach would not yield any advantages here.
That's what I said in more words. I didn't mention the case of encoding the source language in the target because it should be obvious that this is, from the perspective of the translator, the trivial, and as you mention, useless, solution.
It is highly unlikely that this is feasible. I don't know nix in detail but I infer from your post that it is an untyped language. Untyped languages can, in general, not be translated to (statically decidable) typed languages.
> which could then be abandoned.
It could not, unless you convince a majority of the contributors to the package repo to switch to your new language.