[Решено] WPForms Multilangual + WPML: translating validation messages

Background of the issue:
I am trying to translate the validation messages for forms ('this field is required') to Dutch on my site скрытая ссылка. The forms themselves are translated through 'translations', but the validation messages do not show up there. I also can't find them in String Translation.

Symptoms:
Validation messages for forms are not appearing in the translation or String Translation sections.

Questions:
How can I translate the validation messages for forms to Dutch?
Why are the validation messages not appearing in the String Translation section?