Here's the default page: https://formkeep.com/thanks
It's localized in many different languages, so your customers will see it in whatever they're local language happens to be set to.
There's three ways to customize the default thank you page.
Under the Edit tab of your form, you can check the Custom confirmation message section and change the two lines of text displayed on the form by default. It won't be localized to the customers language, but you can make it more friendly.
You can set a Redirect URL, and after the form is collected, we'll send them directly to that url, they won't see the default page at all.