Sadly, that used to work better than it does now. From some versions ago, it was upgraded to be "smarter" and do google searches and link lookups and whatnot. I'm nowadays struggling to get the dictionary lookup shown. That's a shame; as a non-native speaker it's a super helpful feature. I've been trying to disable the other shenanigans, but without much success.

Woah, I found a way to fix it! You need to remove the checkbox "Show Spotlight results in <whatever the feature's name is in English>" in Spotlight settings. I wouldn't thought that I need to go look for settings elsewhere to fix that.

That used to send whatever word (possibly a password, or some other confidential thing) you deep-clicked on out to the lookup server unencrypted. If you happened to be on open wifi (like in a coffeeshop), it would broadcast it totally unencrypted to everyone within radio range. I reported it and now it uses TLS. :)

Tap on a word with 3 fingers.

You can also hover over the word and hit Command-Shift-D for people not using a trackpad.