Maybe this SO thread will help: https://stackoverflow.com/questions/29088590/how-do-i-change-the-language-of-the-checkout-form-in-stripe. Specifically:
The easiest solution is to pass data-locale="auto"to display Checkout in the user's preferred language, if available. English will be used by default. You can also force a specific locale in data-locale as long as it's one officially supported by Stripe.
data-locale="de"