Associative array triggered from Custom Taxonomy (WordPress PODS)

If I understood your question correctly, you want to have the flag span added automatically your post when it is associated with the country custom taxonomy, right? If that is the case, then you could add the following helper functions to your functions.php and then use them in your post template.

The example assumes that the term slug matches the flags css class suffix (eg. United States -> us).

// functions.php
function get_post_locale(int $id)
{
  $terms = get_the_terms( $id, 'your_custom_taxonomy' ); // add correct taxonomy name here
  return ( $terms && ! is_wp_error( $terms ) ) ? $terms[0]->slug : '';
}

function print_post_locale_flag(string $locale)
{
  if ( $locale ) {
    printf(
      '<span class="flag-icon flag-icon-%s"></span>',
      esc_attr($locale)
    );
  }  
}

// single post template
print_post_locale_flag( get_post_locale( get_the_ID() ) );

Of course you can have another helper function to map the term slugs to correct css classes, if you want.