If you just want to log to a text file a highlighted word looked up via the Dictionary pop-up, one option is to create an Automator service and assign a keyboard shortcut to it. The service will create a text file on your Desktop and append to it. The service offers only basic functionality. The word list created may be useful for importing into a vocabulary trainer, but may require some manual pruning.
1. Create a new Automator service. Leave as is "Service receives selected text in any application". Add the following code to a 'Run AppleScript' action and save the service with a name:
on run {input, parameters}
tell application "System Events"
key code 2 using {control down, command down}
end tell
set filePath to "~/Desktop/dictlog.txt"
set selectedWord to quoted form of (input as string)
try
do shell script "cd; echo " & selectedWord & " >> " & filePath
do shell script "cd; awk '!x[tolower($0)]++' " & filePath & ¬
" > dictlog.tmp && rm " & filePath & " && mv dictlog.tmp " & filePath
end try
end run

2. Assign a unique keyboard shortcut by navigating to > System Preferences > Keyboard > Shortcuts > Services. Find the name for the service you created, click "none", then "add shortcut", and press the keys for the desired shortcut.

3. To use, highlight a word by double-clicking on it or by click-dragging the cursor across it and then use the keyboard shortcut you created. A Dictionary pop-up with a definition if it exists will appear and the word you highlighted for the search will be logged to dictlog.txt on your Desktop. Search words will be deleted automatically from the log as necessary to avoid duplication. If you don't want to log a word looked up, don't use the service you created; use your usual method for the Dictionary pop-up: Control-Command-D, a three finger tap, Control-click, etc.
Only tested on OS X Yosemite 10.10.5. May or may not work with other versions of OS X.