It's not a stupid question, Kewin. If something has been bothering you, it's always worth asking. In fact, it's quite an interesting question. We use the first form to refer to someone's ability to speak a language. We also use the first to refer to a native language which is always spoken or the only language which a person is able to speak; we sometimes use the second when someone happens, temporarily, to be speaking in one language as opposed to another. In other words, if you are speaking 'in' a language, you have usually made a decision to speak in that language rather than in another one. For example, you say: Do you speak Russian? They speak Portuguese in Brazil. but you could say Yumiko Suzuki lives in Tokyo, but she speaks in English to her friends when she doesn't want her little brother to understand what they're saying. He's only six, and he only speaks Japanese. As a general rule, it is always correct to say 'speak English'. This is never wrong. For example, in the sentence above, you could say 'Yumiko speaks English to her friends'. This is fine, and it has the same meaning. However, it would be very strange to say that her little brother 'speaks in Japanese'. You would expect a six-year old Japanese boy in Tokyo to speak Japanese. It begs the question 'Well, what other language would he speak?' I hope that makes sense. In return for answering your (not very) stupid question, could you tell us whether it's the same in French? Is there a difference between 'parler anglais' and 'parler en anglais'?

I'd say "speak English" answers the question "speak what?", and "speak in English" answers the question "speak how? / speak using which language?" So, if you "speak in English", you would have certain sentences or information conveyed using English. We'd probably sooner use "say something in English". "Speak English" tells us your choice (or not!) of language.

to speak English = have knowledge