They will. It would be almost impossible to grow up in Scotland today without learning to speak English. Your child will be immersed in English in many social situations, including some within school, unless (s)he attends a dedicated Gaelic school as opposed to a Gaelic unit. From primary three onwards (s)he will be taught to apply the language skills (s)he has learned through Gaelic to English as well, and will quickly become multilingual. It is worth remembering that multilingualism is accepted as normal throughout the world.