Why doesn't El Capitan Spotlight search in custom dictionary?
Well, I managed to almost get it working by going to /Library/Dictionaries and rename Oxford Dictionary of English into something else. Then Spotlight started showing the results from dicts in Dictionary.app.

However, this is a rather kludgy solution for me because:


You have to Press Cmd+L in order to view translation, and there is no way to move 'Definition' up in Spotlight order
After I restarted my Mac, my Spotlight started to display result from another built-in dict, Oxford Thesaurus.


I guess, you could just remove all dictionaries that you do not use.
Anyhow, maybe it could be of some help for you.