@hbhorsesbe, I'll need for you to generate a One-Page Checkout log and send it to my personal email address (which I'll send you via PM).

Turn on the One-Page Checkout Debug via the admin configuration tools. Log in as a customer (and make sure that the language is changed to French). Place an item into the cart and navigate to the OPC's checkout_one page.

Choose a shipping and payment method and, if your store is set up to require one, 'tick' the conditions checkbox.

Using your FTP program, navigate to your store's /logs directory and delete the myDEBUG-checkout-one-xx.log file (where xx is the customers_id associated with the test checkout).

Now, click the "Submit Order" button ... once. The processing, in your case, returns back to the checkout_one page. At that point, copy that generated debug-log and email it to me.