Hello, @hostdz,
1. Can you please share your site WP Admin URL and Credentials with me in a private content area of this topic so that I’ll check and help you out, please share the word with which you want to change/translate as well in French.
2. Checkout page is the default WooCommerce page and you can not edit it directly. To edit this page requires extra customization, you can check the below-given articles and follow the instructions in them.
https://woocommerce.com/posts/customize-checkout-fields-woocommerce/
https://quadlayers.com/customize-woocommerce-checkout-page/
3.. Yes, It is a cache issue, to solve this, first you have to contact your service provider (Hosting) and flush all the server cache and ask them to disable the cache system from the server after that please once try to disable the cache plugins, if you are using them in your site then check back to your issue after clearing the browser cache it will be resolved and after completing the development, you can again turn ON the server cache and site cache as well.
Regards 8Themes Team.