Do the technical wiring to add and expose a language facet

@OC000021124312 I am not trying to make any statement about the schema and/or content of the language (facet) field, but I thought it might be helpful especially for testing if I provide the necessary technical wiring to add and expose a language facet field via the API and internal UI, especially since doing so is a not particularly interesting exercise in copy & paste programming.

Also note that the UI will probably be much more usable if !515 (merged) is merged first to reduce the problem of the facet side-bar becoming ever longer.