"Cool" has been used widely in German slang for several decades now and is generally understood by most people, so one could simply say "cooles Mädchen". Other possible translations of "cool" varies...
The German verb to cool is kühlen, the correponding adjective is kühl - a cool breeze = eine kühle Briese The Germans have also adopted the English word cool in the sense of good, e.g. he's a cool...