Thank you very much, Antechinus.
I have tried that one (and several other things, which did not work):
$txt['tabbedSmiley_tabs'] = ' ';
sceditor-emot-head-tabs>li:nth-child(2):before { content: "Any text you want";}
You will only see the   in all tabs.
With this one, it is working nearly...but the   is also there...
.sceditor-emot-head-tabs>li:nth-child(2):before {content: "Mehr";}