The language that your customer will see in your checkout can be set up in your account.
When you are logged in go to the Checkout app and choose Settings. In the settings your can edit the the language of your checkout. You can either make your checkout available in one language only and thus force your customer to see the checkout in a certain language. Or you can enable multiple languages which will, based on the user agent of the customer, decide in which language the checkout is displayed to your customers. If the language of the customer is not available at payever, the default language option will be shown to the customer.