I found the solution ….
just replace :
// Traditional WordPress plugin locale filter
$locale = apply_filters( ‘plugin_locale’, get_locale(), ‘clean-testimonials’ );
$mofile = sprintf( ‘%1$s-%2$s.mo’, ‘clean-testimonials’, $locale );
by
// Traditional WordPress plugin locale filter
$locale = apply_filters( ‘plugin_locale’, get_locale(), ‘clean-testimonials’ );
$mofile = sprintf( ‘fr-FR.mo’, ‘clean-testimonials’, $locale );