Hi mrpolab,
It is not correct to add <?php echo qtrans_generateLanguageSelectCode(‘image’); ?> to the header. As specified on the qTranslate Slug doc page – the correct way is to add
qts_language_menu(‘image’);
Please, look carefully at the doc on project website https://not-only-code.github.com/qtranslate-slug/ and the same info is in readme file in the plugin.