A theory is an explanation or proposed reason why something occurs, so you can say 'theoretically, man could travel to Mars', but you can either speak a language or not, so there is no theory involved.

It doesn't make any sense to me. Either he can speak English (which he is doing with his question) or he can't. There's nothing "theoretical" about this fact! In common speech, people use this word like this: "Theoretically, we could drive from London to Manchester in 4 hours if we left after the rush hour. But then again, there may be various factors which could make the journey longer like..." You use "theoretically" when speculating about something or talking about possibilities and predictions. The facts are not certain so you want to make clear that your opinion is "theoretical" i.e. based on a theory.