Xapian - searching polish characters (enconding issues?) - internationalization

Question

I am using xapian as a search engine for my website.
Recently I found out it does not search words containg polish specific characters like ś, ą, ć, ę.
Anytime I am trying to search word containg one of those language-specific chars it returns no results. Are there any encoding settings in xapian?
Those are my indexing and searching functions ($document has content, id and route field).
protected function _indexDocument($document, $indexer, $database)
{
$doc = new XapianDocument();
$content = Zend_Json::encode($document);
$doc->set_data($content);
$indexer->set_document($doc);
$indexer->index_text($content);
$term = (string) md5($document['id']);
$doc->add_boolean_term($term);
$database->replace_document($term, $doc);
return true;
}
public function searchDocuments($phrase, $page = 0, $limit = 10)
{
$page = (int) $page;
$limit = (int) $limit;
$database = new XapianDatabase($this->getDatabasePath());
$enquire = new XapianEnquire($database);
$qp = new XapianQueryParser();
$stemmer = new XapianStem("english");
$qp->set_stemmer($stemmer);
$qp->set_database($database);
$qp->set_stemming_strategy(XapianQueryParser::STEM_SOME);
$query = $qp->parse_query($phrase);
$enquire->set_query($query);
$matches = $enquire->get_mset(($page-1) * $limit, $limit);
$documentCount = $matches->get_matches_estimated();
$i = $matches->begin();
$documents = array();
$rawDocuments = array();
while (!$i->equals($matches->end())) {
$n = $i->get_rank() + 1;
$data = $i->get_document()->get_data();
$documents[] = $this->_prepareDocument( Zend_Json::decode($data), $phrase );
$rawDocuments[]= Zend_Json::decode($data);
$i->next();
}
$pageCount = ceil($documentCount / $limit);
if ($page > 0) {
$prevPage = ($page - 1) * $limit;
} else {
$prevPage = 0;
}
if ($page < $pageCount) {
$nextPage = ($page + 1) * $limit;
} else {
$nextPage = $pageCount;
}
$result = array('results' => $documents, 'results-raw' => $rawDocuments, 'paginator' => array(
'page' => $page, 'limit' => $limit, 'pageCount' => $pageCount,
'prevPage' => $prevPage, 'nextPage' => $nextPage,
'documentCount' => $documentCount));
return $result;
}