Stripe currently supports 12 languages, 25 countries and 139 currencies. WP Simple Pay lets you set the locale (language) for your checkout overlays as well as the preferred date format, currency and currency format for your site’s region.
French/Euro One-Time Set Amount
Here’s a demo with an alternate locale (French), date format, currency (Euro), and currency symbol position (right).
French/Euro One-Time Custom Amount
Here’s the same locale and currency settings applied to a custom amount input.