To resolve your problem you should rename the files to:
it-l10n-better-wp-security-fr_FR.mo
it-l10n-better-wp-security-fr_FR.po
It is also recommended (but not mandatory) to copy these 2 files to a wp-content/languages/plugins/better-wp-security folder. You will need to create this folder as by default it does not yet exist.
This way when the iTSec plugin is updated in the future you will not lose your translation files … nor do you need to copy the 2 files again to the wp-content/plugins/better-wp-security/lang folder after updating …
Are you willing to share your French language files ? If so please send them to [ redacted, support is not offered via email, Skype, IM etc. only in the forums ]. Merci !
dwinden