The language of the checkout is determined by the locale of your WordPress installation.

Change the language of your checkout page through the following steps:

1. Log in to your WordPress Admin.
2. In the left-hand menu, navigate to Settings → General  (WordPress settings, not WooCommerce).
3. Change the Site Language option to the language of your choice.
4. Click Save Changes.

You might need to clear the site cache if your changes are not visible.

Note: If you have made any language-related changes, either via a third-party plugin or a snippet, you need to make sure the WordPress locale is set to the desired default language of your site for the above changes to work.