+1 I do not see Kadence in the string translation area either.
I was able to solve this in a different way. The answer depends on the type of setting you’re trying to customize for the language.
For buttons/text in the Kadence header/footer, I used the plugin Add Polylang support for Customizer.
For Kadence Elements, I had to enable translation for the Elements content type in Polylang settings. Then I made translated versions of my Kadence Elements. Because I was using the element in the footer by placing a widget with a shortcode, the string translation area showed a widget group and I updated the Element ID in the shortcode for each language.