UserAgent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_4) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/11.1 Safari/605.1.15

Steps to reproduce the problem:
1. Go to a page with text written in a language that doesn't use spaces to separate words.  data:text/plain;charset=UTF-8,今日は涼しいね is an easy example
2. Perform the command you have set for dictionary lookup in System Preferences (Force click on Macs that have it, three finger tap on Macs that don't) with your mouse over 涼しい

What is the expected behavior?
The dictionary lookup is performed on 涼しい, yielding a result if you have a Japanese dictionary enabled

What went wrong?
Depending on which character your cursor was over, the dictionary lookup will happen on various different parts of the word but never the whole word.  Unsurprisingly, these lookups won't give the definition you're looking for.

Did this work before? No 

Chrome version: 63.0.3239.132  Channel: stable
OS Version: OS X 10.13.2
Flash Version: 28.0.0.137

This works properly in Safari.  In addition, if it happens to mess up, Safari will always look up a highlighted section if the cursor was over some highlighted text, so even if it doesn't work with the default parsing, you can always highlight the word you wanted to look up and then perform the dictionary lookup action.  This also does not work in Chromium.