Hi leahwatchorn.
Being able to switch between English and French is one of the ways QuickBooks Online lets you customize your experience to better suit you and your customers. I'd be happy to go over if you can switch language format after an invoice has been sent.
You can switch between English and French in product at any time but to switch the invoices language you'll need to switch the customers language as well. If you switch the customers language all invoices will change to the language selected including the one that has already been sent.
If you have any questions let us know and we'd be happy to help.
