[tzs]

I think that to earn the title "Eternal Language" that should have to work in both directions.

If I am an expert in language X and take a solar powered laptop and go spend 10 years living as a hermit in some isolated place while working on some big X program, with no communication with the outside world other than idle non-technical chat with the people of the village I go to monthly to buy supplies, if X is an Eternal Language then

1. My code that I wrote as a hermit should build and run on the outside world's systems,

2. I should still be an expert in the language X, only needing to learn library changes, fashion changes (such as coding style changes), and tooling changes to be ready to take a job as an X programmer at the same level as I had before I became a hermit.

Alternatively, suppose I don't know X but wish to learn it. To earn the title "Eternal Language" I should be able to go into my library and read the "Learning X" book I bought 10 years ago but never got around to reading and that should mostly be equivalent to buying and reading a recently published book on X.