catalog/language/en-gb/common/maintenance.php

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<?php
// Heading
$_['heading_title']    = 'Maintenance';

// Text
$_['text_maintenance'] = 'Maintenance';
$_['text_message']     = '<h1 style="text-align:center;">We are currently performing some scheduled maintenance. <br/>We will be back as soon as possible. Please check back soon.</h1>';