Language
Ibiza is catalan territory, that means that Catalan is the official language. But, Castillian Spanish is widely spoken. If you arrive to Ibiza with English as your only language you will be perfectly fine. In all hotels, bars and restaurants the workers managed to speak good English. However, if you are in places which do not rely on tourist income, you may find it handy to know a little bit of tourist-Spanish to get by.
Those who work in the supermarkets do not speak much English, so you have to expect to speak Spanish to them in order to get the right type and amount of meat in the meat-department of the Supermarket. If you do not know any Spanish at all you will manage by pointing and explaining with your fingers, so, no need to worry.
Also, it is handy to know a little bit of Spanish, the service and attention from waiters and staff gets a little better when you make an effort to speak to them in their own language.