Implement Language Fallback on Solr Index Level

We should implement a fallback from a language variant without existing configuration to another language code that has one, or to undefined if none can be found.

For example, assuming that "en" and "de" are configured:
"de-at-custom" -> "de"
"de-at" -> "de"
"talk-like-a-pirate" -> und

TODOs:
* Remove some old code and configs
* Implement language fallback as described, especially make sure it does not crash when a new language without default configuration is added but not configured (yet).
* Add tests