For me, to "Speak English" means that you know and speak the language just like it says. "Speak in English" tells me that the person knows more then one language and you would ask them to speak in English as opposed to the other language. However, there is hardly any difference, its just a slight difference in expression. You can say "talk in English" but, as a general rule, "talk" tends to mean a back and forth communication between more than one person, and "speak" tends to emphasize the actions of a single person.