How to modify default language

Hi,
i want to modify original English language in my Drupal installation. For example; to change search button label in $search_box text. Is there any way to change default English words / sentences. I tried to use search_box function in template.php. But it couldnt work.

function search_box(&$form_state, $form_id) {
$form[$form_id] = array(
'#title' => t('Search this site'),
'#type' => 'textfield',
'#size' => 15,
'#default_value' => '',
'#attributes' => array('title' => t('Enter the terms you wish to search for.')),
);
$form['submit'] = array('#type' => 'submit', '#value' => t('Custom Search'));
$form['#submit'][] = 'search_box_form_submit';

return $form;
}

Thanks