Add full support for search in multi-language ZIMs

I'm opening this issue to open the discussion. I'm not convinced yet this is something we should / can support.

Since quite long, ZIM specification has the Language metadata which can be a list of languages, opening the road to have multiple languages in one ZIM.

TED ZIMs are typically using this feature, allowing to display localized video title and description when available. There is only one video stream, one audio stream, and multiple localized subtitles.

https://github.com/openzim/overview/issues/38 focuses on better indicating importance of language in each ZIM, by providing details.

This however does not overcome another significant limitation which is the search.

We cannot search in one specific language, and either add "fake" items in the search index at the risk of getting duplicate results, or just add one item (as done today) in native language of the video, meaning limited possibilities to find the video we are searching for.

I do not (yet) have much proposition in how to move this forward, this would be a significant software change anyway.

No files, tests, or entry points are named. Start by reviewing the ZIM Language metadata and the discussion in overview#38, then determine whether language-specific search should avoid duplicate results; the issue currently has no agreed implementation or definition of done.