The find queries come from the current language context but you can override https://world.episerver.com/documentation/developer-guides/search-navigation/Integration/cms-integration/GetContentResult-and-GetFilesResult/ and as you can make multiple searches in one request https://world.episerver.com/documentation/developer-guides/search-navigation/NET-Client-API/searching/Multiple-searches-in-one-request/ I would suggest perhaps making your search request with a different request per language using the explict language request. Then you can fallback to the language you want where needed.
Unifed search does also support PublishedInLanguage which you can see here https://world.episerver.com/forum/developer-forum/EPiServer-Search/Thread-Container/2015/2/episerver-find/
Hi,
I have language A and fallback to B so i don't know Episerver can search If content is not available in A then i want to fallback to B
Regards,
Thang Le