We're really sorry, but you can't choose the font-family for individual comments. This restriction is made by WordPress, not the wpDiscuz side. 

In this case, you should use the CSS code below to change the whole comment section font-family. Please watch this video: https://app.screencast.com/ENGSSL7DUoBxd

#wpdcom p, #wpdcom div, #wpdcom span {
     font-family: Pontiac !important;
}

Put the code in the "Custom CSS code" textarea, located in the Dashboard > wpDiscuz > Settings > Styles & Colors admin page. 

Don't forget to delete the caches before checking.

Some symbols are only present in the Pontiac Font

Τέρεν κ’ εμέν, οίραχο! Τ’ εμόν η φορεσία άσπρον άμον το ον εν και τιζ’ άμον τη ναϊνάν.

the comment should look like this