The easiest way would be to designate French for the language before you start the download from Adobe. But, since that was not the case, then uninstall and reinstall of the installation files would seem justified. I am not sure if just changing the country asked for in the installation will do the job. Look carefully for other opportunities at the beginning of the install for setting the language to be used in the program.
In support of what I have just written, please see Premiere Elements FAQ
The following is a copy/paste of the FAQ related to your question
I would like to install my product in a different language than the one in which I installed the product during my trial period. Can I uninstall and reinstall my application in a different language?
Photoshop Elements and Adobe Premiere Elements offer the option to install the application in any one of many languages available. If you would prefer to install a purchased product in a language other than the one you installed during your trial period, simply go back to the original installer (whether DVD or Electronic Software Delivery), install the correct language, and then enter your serial number.
Here I am assuming that you can do this with the tryout before purchase as well as after the purchase.
Please review and consider.