First thing first! English is widely spoken in Costa Smeralda due to its long tourism experience so you'll find no problem at all.
Costa Smeralda is the real deal for you because it has a variety of clubs and discos, smaller and bigger ones for people your age (that is also mine).
It will be best to book and hotel like The Sporting, located almost in the town centre of Porto Rotondo, with a direct access to the sea and from which you can enjoy the nightlife.
Even if you are staying 5 nights I suggest you to stay in one place not waisting time transferring form North to South. If you really want to visit al least two places then you can go to Alghero for 2 nights.
Alghero is not too far from Costa Smeralda and it has a good nightlife (mainly restaurants and classy clubs plus one or two discos).
Hope it will help
Fort further information I'm at your disposal