Simple config with a `langcode` element, but no translatable elements, should raise a validation error

Problem/Motivation

This is spun off from 📌 Require `langcode: …` only for simple config that contains translatable values RTBC .

In that issue, we trigger a deprecation when validating simple config that has no translatable elements, yet still contains a langcode key. Having a langcode for non-translatable config doesn't make sense, but for historical reasons it probably exists in the wild. That's why it's deprecated.

In this issue, let's turn that deprecation to a full-fledged validation error.