To translate a theme you need to edit .po and .mo files.
they should be in /languages/ or /lang/ folder of your theme.
If you don’t have your language, then :
Open en_US.po with poEdit and save the file as fr_FR.po
or duplicate rename it, then open it with poEdit as you wish.
Go to catalogue/configuration, and Put french & france
Make your traduction
save the file
you now have an fr_FR.mo file (and fr_FR.po automatically generated)
Put it in the /languages/ or /lang/ folder of your theme if you did not edit from there.
Now, in your
wp-config.php be sure to have then
fr_FR on the lang line.
french is choosen in your blog options.
French was used but of course just choose the language you want traduction for.