(Most web browsers will automatically offer to translate the page depending on the viewers language settings)

To edit this manually, you'll want to click your campaign and click the Design tab > then click Popup Design like so: 

Using French in the example below. You would start by copying the default text on your forms, and paste this into a translator (if needed) so that it reads like this: 

You can click any of these text elements to translate them into another language: 

Altogether it will look something like this: 

You can also change the email field so that Email is read in another language as well. This is done on the Form tab of your campaign > by expanding the Form Fieldssection:

Don't forget to translate the Thank You page! 

To make this change you'll want to edit the following section, located at the bottom of the Design tab: 

Success! You have translated your campaign.