StartOfTheWeekGlobalFluentForms.php

// add extra settings to flatpickr calendar
function bleye_ff_custom_calendar_settings( $config, $settings, $form ) {
    $start_of_week = get_option('start_of_week');   // get the option from the WP settings
    $config['locale'] = ['firstDayOfWeek' => $start_of_week];   // add the config to flatpickr
    return $config;
}
add_filter('fluentform/frontend_date_format', 'bleye_ff_custom_calendar_settings', 10, 3);