They are very much full of italians - who are all seated in the restuarants together. MSC tend to put same nationalities together (for what purpose I dont understand). Be prepared to witness pushing, shoving and rudeness.
Brits are definately in the minority. Mainly Italian and German; some Spanish. Europeans do not know how to form an orderly queue - expect a free for all with lots of pushing and shoving. Even standing right at front of reception desk they manage to push infront. They usually appoint an english speaking host/hostess to look after you and when we were on MSC at xmas I think there were only around 50 to 70 Brits. Everything is announced in five languages and shows tend to be more circus type so all can enjoy.
There will always be a lot of Italians. depending on where they are sailing you sometimes get other large groups eg in the baltics you also get a lot of Russians and Germans (they allow different nationalities to embark/disembark at different ports of call on different days from the start and end dates publishe in the UK). In the caribbean there will also be a fair number of Americans. Brits tend to be in the minority, although on a new Year repositioning voyage to the caribbean a few years ago there were about 400 of us. In the med there will also be a lot of french and Spanish to.
Two can play at that game!
Thanks for your answers on MSC nationalities - sounds the same as Costa.