I'm not sure this should be a hard requirement before expanding beyond the ~30 languages with built in stop words. I certainly agree we should do it though.

Hmm, I wonder: http://www.elasticsearch.org/blog/stop-stopping-stop-words-a-look-at-common-terms-query/

I believe that is what nemo was referring to. The problem (right now) is that was use query string queries rather than term queries. For what we do, it makes a lot of sense. Anyway, query string queries don't play nice right yet with common terms queries. They could possibly be made to but I'm not sure about that yet. It'd probably make more sense to make this change in elasticsearch and for us to just flip the switch to turn it on.

I don't know anything about implementation details but yes, that would seem the most elegant way to handle it from the small hints I gathered around. However, it may also be viable to automatically generate "standard" stopwords lists for each language, from what I understand.

https://github.com/elasticsearch/elasticsearch/pull/5005

The upstream pull request linked above was closed in March 2014. Is there anything else which needs to be done upstream?

Removing the Discovery tags - it looks like this has been done for the most part.