When you ask someone to `speak in English’, what you want him to do is to use the English language. Perhaps he is speaking in a language that you don’t understand, and therefore you want him to talk to you in English. This is the language that you want him to use on this particular occasion. When you say that he `speaks English’, what you mean is that the individual knows and has the ability to use the language. In India, for example, many of us can speak two or more languages, and depending on the context we choose to speak in a particular language.
*The CEO speaks five languages. At the press conference, he spoke in English.