Hi @Jakub, 

Regrettably, the specific field you have highlighted “This field is required” is actually hard coded into all Klaviyo sign-up forms as English. It is not currently possible to adjust the displayed message in a different language.

If you want the message to be in another language other than English, you will need to create a custom form utilizing our subscribe API.  This will allow you to code and design your custom signup form however you want via HTML and the form will submit to the new frontend subscribe API. 

In the mean time, I have submitted a feature request on your behalf!

Thanks for participating in the Community!

-Taylor