I just run in to the same issue AND fixed it.
The issue starts on line 82 of search.php
$aLinks = array(
'simple' => array('href' => $sUrl . '?search_mode=sim', 'title' => _t('_search_tab_simple'), 'onclick' => '', 'active' => $bSimAct),
'advanced' => array('href' => $sUrl . '?search_mode=adv', 'title' => _t('_search_tab_Adv'), 'onclick' => '', 'active' => $bAdvAct),
'quick' => array('href' => $sUrl . '?search_mode=quick', 'title' => _t('_search_tab_quick'), 'onclick' => '', 'active' => $bQuiAct),
);
The solution is the following:
$aLinks = array(
_t('_search_tab_simple') => array('href' => $sUrl . '?search_mode=sim', 'title' => _t('_search_tab_simple'), 'onclick' => '', 'active' => $bSimAct),
_t('_search_tab_Adv') => array('href' => $sUrl . '?search_mode=adv', 'title' => _t('_search_tab_Adv'), 'onclick' => '', 'active' => $bAdvAct),
_t('_search_tab_quick') => array('href' => $sUrl . '?search_mode=quick', 'title' => _t('_search_tab_quick'), 'onclick' => '', 'active' => $bQuiAct),
);
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