I also tried to translate the Title Categori and Link, and I did find the right strings, but when I translated them, it did so the text in the columns didn't load. So if I translate Title, then all the titles did not load, but the rest did.
To translate the text Title, Categories and Link, please go to WPML > Theme and plugins localization > Scan strings in the plugin "Document Library Pro".
Then go to WPML > String Translation > Search for strings under the domain "document-library-pro" and translate them.
Please check this screenshot on the staging site hidden link