Rotterdam is a Dutch city, so the primary language spoken in the city is Dutch. But Dutch children learn to speak English already from an early age, in some schools they already start giving English lessons to children at the age of 6. As a matter of fact The Netherlands even tops the list of the English Proficiency Index. You can start an English conversation with pretty much any one who went to elementary school in The Netherlands. Some speak it better than others but most people can answer basic questions and hold a conversation.
Next to Dutch and English a lot of other languages are spoken in the city, considering Rotterdam is a very international city. People from close to 180 nationalities are found within the city, of which the largest minorities are Surinamese, Turkish, Moroccan, Dutch Caribbean and Cape Verdean.
So do they speak English in Rotterdam? Most people do, but not as a first language.