Change the language on Stripe gateway of JetFormBuilder. In 'locale' => 'da', change 'da' to your language code.

change-stripe-locale-jfb.php

<?php

add_action(
    'jet-form-builder/gateways/before-create',
    function ( $request ) {
        $need_request = \Jet_FB_Stripe_Gateway\Compatibility\Jet_Form_Builder\Actions\Create_Checkout_Session::class;

        // We check whether this is a request to create a new payment in Stripe
        if ( ! is_a( $request, $need_request ) ) {
            return;
        }

        // We initialize the settings before the request. It is important
        $request->get_request_args();

        /**
         * Here we can add the parameters we need. All their types are listed here
         *
         * @link https://stripe.com/docs/api/checkout/sessions/create
         */
        $request->set_body(
            array(
                /**
                 * Here you can specify your locale
                 *
                 * @see https://stripe.com/docs/api/checkout/sessions/create#create_checkout_session-locale
                 */
                'locale' => 'da',
            )
        );
    }
);