But that does not allow you to manipulate it automatically (think about moving domains or something like this), nor does it help you to detect errors (which few tools do, but many actually should do).

So now you need the runtime for that other language in your system-wide configuration management solution. Plus all the other runtimes for languages someone decided to use as a configuration language.

Sounds alright to me, as long as you restrict to sane, reasonable languages. For example lua's "runtime" fits in a single .c file and is likely simpler than many xml or json parsers.