Popup strings not translated

Hi,

I’m using the coupon box plugin in combination with WMPL. Somehow I don’t see how to translate the content of the popup (and bottom bar) to other languages. I created everything in Dutch. That’s the default language of my site. I tried searching for strings in WPML I tried using the theme customizer in various ways: first select language from admin bar and then open the customizer, and tried opening the customizer and then do the language selection on the page.

What would be the correct procedure? Multi language is enabled in the plugin settings.