You have to make your own localization. Download the theme again and upload the zip file to this site, then download the generated .po file.
Edit the .po file with Poedit and translate all text strings. Then save it as fr_FR.po and fr_FR.mo.
Crate a new folder in the Serene theme folder called “languages” and move these two files into the folder. Upload it all with FTP to your server.