Great tip on overriding Drupal’s required string using form api. Just put into your theme’s template.php:
<?php
function THEME_form_required_marker($variables) {
// This is also used in the installer, pre-database setup.
$t = get_t();
$attributes = array(
'class' => 'form-required',
'title' => $t('This field is required.'),
);
return '<span' . drupal_attributes($attributes) . '>(required)</span>';
}
?>
Source and further reading: http://knackforge.com/blog/selvam/drupal-form-api-overriding-required-field-asterisk