If your theme does not include the Magento's multi language translation functionality in the checkout, the Braintree module won't be able to display the Card or PayPal form fields.

This error typically can be seen from the browsers console window as such -

There will be missing a piece of code from the theme template on the checkout, that looks something like the below -

<script type="text/javascript">//<![CDATA[
        var Translator = new Translate({});

The following JavaScript file may also be missing, and will need need including on the checkout page -

<script type="text/javascript" src="[url]js/mage/translate.js"></script>

Once the JavaScript code and file have been added to the checkout templates, the Braintree module should be able to display the Card & PayPal Forms.