Well – you did not tell me the file name of your WordPress language file as I asked so I will have to make the assumption that it is named ‘fr_FR.mo’.
And if that is correct then Simple:Press language files should have the name of the component (i.e., ‘sp’ fpr the core file) followed by a dash and then the Wp language file name which would be – if my assumption is correct – ‘fr_FR’.
So – the final file name of the core file would be ‘sp-fr_FR.mo’.
Note however that this requires the .mo file NOT the .po file. It is the .mo file that contains the translations in a format that WordPress can make use of. If you downloaded the Po files then you need to go back and download the actual mo files.
Also note that the core ‘sp’ file does not really contain that much as the majority of the translations are made via the Sp theme you are using. The easiest way to discover whether the core ‘sp’ language file is working is to look at the ‘about’ popup from the footer.