Custom Error messages in Form Submission

Hi, I am working on a clients` website which is in an Eastern European language so a lot of custom translation to do.

One thing where I have hit the wall is custom error messages in form submission. 

Basically I want to do what this often suggested post does: https://www.silvabokis.com/squarespace-tips-archive/dynamic-translation-of-squarespace-form-error-messages

But it does not seem to work at all. 

Some have mentioned that since a recent update the guide above does not work any longer just wondering if that is true or do I need to do some troubleshooting?