When using the query-mode (the language is noted as an attached ‘?lang=en’ in the URL) a performed search shows the results in the default language (which could be the one selected in the browser settings, not the one chosen by a user).
Add the following to ‘Searchform.php’ in ‘design’ -> ‘Editor’
just in front of the ‘<\form>‘ tag:
<input type="hidden" name="lang" value="<?php echo qtrans_getLanguage(); ?>">
… this way the language-parameter will be added to the URL as a hidden field.