Language problem on form

how do i change the language on my form so that when people fill it out it doesn't  give them a warning in german?

On the form that you provided Form warnings are edited/translated. 

If you want to use default form warnings you will have to replace custom massages that are added with original message, like this.

You will have to do this for all warnings that were changed. 

Inside of this guide you can find how to change form warnings:

How to Change Form Warnings

Hope this will help. Let us know if you need further assistance.