-- I need to practice for my English conversation lesson. -- OK. Let's speak English then!
-- I think those people at the next table are listening to what we are saying. -- In that case, let's speak in English! That way they won't know what we are talking about.
Yes. They are interchangeable, but I gave the contexts in which a native speaker might be inclined to choose one in preference to another. (Other native speakers might choose differently, but I think most would find these very appropriate within the contexts illustrated.)
The difference is extremely subtle. speak [language] is a non-contrastive do speaking using [langu