Hi, I try to change in the software the field: "This field is required." into German, but I don't see an option. This message pops up when someone submits the form and didn't fill in the required field.

Do you know where I change this message into German?

I do believe that's in the myfromname/common/js/jquery.validate.js.

You can do this inside the form builder too. Once you have inserted a field that is required and made it so, you scroll down the right hand panel and find a field for Error Message. Write your translation there.
I have done this with Norwegian language, works fine.