What I really want is being able to index documents in multiple languages. Not all my users use the same language, and I don't want their documents and queries to assume English (for stop words, stemming, etc). This is a limitation of most search libraries at this point.

You have a big list of separate libraries providing support for a variety of languages? Great. Unfortunately that doesn't help me make a real multi-language app though. Doing that work right now, with multiple indexes and routing the query, seems very difficult.