Dictionaries.app only has Wikipedia on High Sierra
Had same problem as jakethedog, only Wikipedia showing! Looked through potential blocking of outgoing calls by Little Snitch. When I removed blocking of nsurlsessiond I got all this other dictionaries also.

Answer to Allan below: Well this is relevant only if you use software Little Snitch for privacy purposes to block outgoing calls from different software https://www.obdev.at/products/littlesnitch/index.html
Sometimes one blocks outgoing calls with unexpected consequences. In my case I blocked Apple process nsurlsession from "calling home" since it is not entirely clear what info this process transmits to Apple. Apparently it also blocked discovery of these dictionaries. Little Snitch users will understand, for others it is not really relevant.