No need to create legend for that.

Proceed in following way.

1. register your email template

add_filter( 'fep_eb_templates', function( $templates, $where ) {
  $templates['new_message'] = 'Nieuw bericht';
  return $templates;
}, 10, 2 );

2. Now in emails/new_message.php file use

$language = get_user_meta( fep_get_userdata( $user_email, 'ID', 'email'), 'icl_admin_language', true );

if( 'fr' == $language ){
    require( fep_locate_template( 'emails/new_message_fr.php' ) );
}

Let me know.