Hi @pipoulito:
<?php
$queried_object = get_queried_object();
$taxonomy = $queried_object->taxonomy;
$term_id = $queried_object->term_id;
$image = get_field('flag', 'country' . '_' .$term_id );
if( !empty( $image ) ): ?>
<img src="<?php echo esc_url($image['url']); ?>" alt="<?php echo esc_attr($image['alt']); ?>" />
<?php endif; ?>
Code’s untested