No translation possible for String 'Message to be sent'

There is no translation possible for the String 'Message to be sent', because the 't'-function is not used.

Line 274

Fix:

$form['tellafriend_message'] = array('#value' => filter_xss_admin("<strong>".t('Message to be sent').":</strong><br />" . nl2br(t(variable_get('tellafriend_message', 'You are invited to check out !sitename at !sitelink'), $placeholder_values)) . '<br/>'));