App for macOS to add words to personal vocabulary list?
You can create an Automator Quick Action that takes the selected word as input and saves it to a predefined document. The Quick Action can then be configured with a keyboard shortcut of your choosing.
Setting it up

Launch Automator (located in /Applications).

Create a new document of type &quot;Quick Action&quot;:


Search for &quot;shell&quot; and drag the &quot;Run Shell Script&quot; action to the right panel:


Configure it to receive selected text from the current application and add this script, modifying ~/Documents/Vocabulary.txt to your needs (see below for a fancier script):
if [ &quot;$@&quot; != &quot;&quot; ]; then
    echo &quot;$@&quot; &gt;&gt; ~/Documents/Vocabulary.txt
fi



Save the Quick Action with a meaningful name like Save to Vocabulary List:


Open System Preferences&gt;Keyboard&gt;Shortcuts, select Services from the list on the left panel, enable the Quick Action and set a &quot;complex&quot; keyboard shortcut, for example ControlShiftCommandV, to reduce the change of a conflict with an existing one:


Open an app, select some text, press ControlShiftCommandV and the text will be added to the file configured in the Quick Action (~/Documents/Vocabulary.txt).


Going fancy
You can easily extend the script's functionality (basic scripting knowledge required). For example, to add a timestamp and a link to the Dictionary app, use this script instead (note that the vocabulary list is now saved to the HTML file ~/Documents/Vocabulary.html):
if [ &quot;$@&quot; != &quot;&quot; ]; then
    timestamp=&quot;$(date +%Y-%m-%d' '%H:%M:%S)&quot;
    dictlink=&quot;dict://$(echo $@ | sed 's/ /%20/')&quot;
    ahref=&quot;&lt;A HREF=\&quot;$dictlink\&quot;&gt;$@&lt;/A&gt;&quot;

    echo &quot;$timestamp - $ahref&lt;/br&gt;&quot; &gt;&gt; ~/Documents/Vocabulary.html
fi

When opened in Safari, the vocabulary file looks like this:

and you can click the word to look it up in the Dictionary app.