Add option to index default translations of entities not available in the requested languages

Does the “Language (with fallback)” maybe already provide the functionality you want? This indexes each translation just once, but then matches also the default (or other fallback) translation at search time when a translation in the desired language is not available.

No it doesn't. We have separate search indexes per language so we need the fallback to happen add index time, not search time.

Never mind, I was able to make it work this way by also indexing content in the fallback language and filtering by language during search time.