Translate config at runtime (aka MultiLangNG in core)

Problem/Motivation

When working on #2806009: Installing a module causes translations to be overwritten, the module MultiLangNG was created: https://www.drupal.org/project/multilangng

It translates config at runtime and thus avoids a lot of problems that core's syscing of string translations and config overrides has. Also it brings additional benefits like contextual translation of config strings.

Proposed resolution

- Postpone this for now, until some international agencies have worked with and validated the approach.
- Move the approach to core.

Remaining tasks