Did you know you can customize the form to give messages in another language? Check this out.
This also changes the submit button language.
Go to Marketing Activities.
Select your form and click Edit Form.
Under Form Settings, click Settings.
Select the Error Language you need.
Click Approve and Close to apply and save changes.
The form must be approved to be used on landing pages.
Remember to approve the landing page draft created by the form changes.
And voila! The form validation and error messages will now be displayed in your preferred language.