Regarding language, these days the second language of the young people working in the tourism industry is English. French seems to be a part of history now so you may encounter older people whose second language is French but on the tourist trail you'll be fine speaking English. The signs you need to read are generally names of places, are bilingual or in our script, not a problem.
The Blue Mosque is in Istanbul, the Hassan II Mosque is in Casablanca, never heard it referred to as Blue Mosque.
Joanna, given you have just 10 days, unless you opt for a tour that includes the camel trek which would facilitate it, I'd stick to Fes and Marrakech from which you arrange to go onward to Essaouira. Whether to spend a night in Casa would depend largely on the time of your arrival there, your proposed mode of transport and time of year. I'd need more information to advise you specifically. Caroline's information is hypothetical and what you need are specifics which I'll be glad to give once you've made some basic plans.
If you arrive in Casa and still have enough hours of light for an onward journey and decide to go on to Fes, for instance, you can take the train right from the airport. You'd change trains at the main Casa station called Casa Voyageurs and go on to Fes. If it's too late to arrive in Fes in daylight there's a hotel at Casa Voyageurs called the Ibis. It's not a great hotel but it's perfectly adequate for a night and has the advantage of being right across the parking lot from the station. There's also one very near the station in Fes which could be another option should you have a late arrival there. I've stayed in both of them and they're very handy if not luxurious. Booking ahead is essential as they're often full.
In any case, I'm sure you see what I mean about making some basic decisions. Once you've done that the rest will fall into place. Something like 4 days each in Fes & Marrakech and 2 in Essaouira would be a great trip.