[AtlasBarfed]

Does a "configuration language" specifically incorporate features for "overlaid" or "unified from parts" configuration?

Much like layered dockerfiles, mature configuration often comes from several places: env vars, configuration appropriate for checkin to git (no secrets), secrets configuration, and of course the old environment-specific configuration.

All of that merges to "The Configuration".

Also, these seem close to templating languages.

I've done this several times with a "stacked map" implementation (much like the JSP key lookups went through page / session / application scopes, or even more convoluted for Spring Webflow.

[q3k]

Answer for Jsonnet: layering/overrides from multiple sources: yes, as you define it (it's a programming language, the logic is yours to define, based on your particular usecase). But no access from environment variables, as that's inpure, and not really in scope for them.