Thanks, this was great find!
I just went thru the the code of acf_form processing and came here to suggest a new hook, but you’ve found existing one!
This works even if you don’t use Polylang, just make sure translations are in your .mo & .po files.
add_filter('acf/prepare_field', function ($field) {
if (!is_admin()) {
$field['label'] = __($field['label'], 'textdomain' );
}
return $field;
}, 10, 1);