So guys, how to fix this bug?
I though it was just on the beta and preview, but it is still bugging me on the official Yosemite.
My customised dictionaries work just fine in Spotlight with all the versions up to Mavericks.
On Yosemite, four out of five dicts work flawlessly in Spotlight, but the main one (English-Vietnamese) doesn't. When this dictionary is selected as a default in Dictionary, Spotlight lookup keeps spinning for ages (see the image).
Image
I'm lured by Spotlight's new look and power, which drive me to upgrade to Yosemite. It has a new look. Oh yes! But is it powerful? Oh la la!