First, make sure you have French language package. I'm not sure if French language is bundled with Q2A. So you must search and download that French language package. It's nothing but a folder with some files in it. You will copy that folder to qa-lang folder.
In case, you cannot find one. You have to develop your own by cloning the default English package. Rename it as something like Fr-fr. Change the basic reference information in metadata.json file, and edit all language files in that folder.
Now, you can log in as admin, and in the first tab (General), you can select French as the language of your Q2A site.