I've sailed Costa twice, once on the Marina and once on the Classica. We did a different itinerary on the Marina (either Greek Islands or Baltic, can't remember which was which ship).
When sailing in Europe, the passengers are mostly European, since Costa's headquarters are in Italy and they market heavily to Europe. On our cruise, fewer than 10% of the passengers were English speakers. All announcements are made in 5 languages, and the ship's personnel are all multi-lingual. The entertainment at night is not language based, although the introductions are also done in several languages! There are cabaret-type shows, singers, we even saw a wonderful puppet show.
We enjoyed the atmosphere a lot, and got to talk to many people from many other countries. The one thing we noticed was how many smokers there were onboard. On our particular cruise, we had a problem with a lot of vibration in the dining room, but this was several years ago and I think that problem has been fixed. We enjoyed the food although it wasn't what I'd call "gourmet," but was certainly tasty and varied.