Hy!
Look here:
/qa-include/qa-page-search.php
Line 90: $qa_content['title']=qa_lang_html_sub('main/no_results_for_x', qa_html($inquery));
The variable no_results_for_x is defined in your language-file i.e. /qa-lang/de/qa-lang-main.php
Hope that helps a little bit.
Marcus