I am also a female in my early 20's and just got back from my second trip to Europe. My first was 5 weeks and this past one almost 2 months.
First I would suggest to start out in London, everyone speaks english, there is a ton to see, and its an easy city to ease into traveling. From there I would suggest Paris, and then heading down to Switzerland/Italy. As others have said do not worry too much about the language barrier, I find a simple hello or thank you in the local language is good enough.
I would suggest to stay away from Marrakech! I spent 4 days in June as a single female, and while I was expecting exotic, Marrakech was the one city I just didn't like. Its too difficult to travel there as a female alone, and I think there are many other more wonderful places in Europe.
Also try not to do too much, leave about 1 day for each week you travel as a down day to relax, read a book or do laundry.
I think you should start with London and Paris if you have alittle more than a week. Two or more weeks add Venice and Rome. Three weeks add Florence or Cinque Terre and four weeks maybe a stop in Switzerland.
If your concerned about the language barrier Switzerland is ideal, as almost everyone I met spoke english!