Don't know about their drink costs but would imagine they'd be in line with the mainstream lines.
In the Med, which is Costa's primary market, it's around 80% European passengers. As many as 6 languages for announcements. In the Carib there is a higher percentage of U.S./Canadian passengers but still heavily European. I don't know what the mix will be in the far east. The staff will certainly speak english, most of the world does. If you're wanting a cultural experience with many people from various parts of the world it's going to be a great cruise for you. If you're looking for an Americanized experience and primarily American culture you need to keep looking. By westerners, I'm assuming you mean U.S., english speaking people onboard/at your table.
Want a great cultural experience? Costa will provide it. Want America on a ship? Why...?