Of course there is a difference between to know a language and to speak it. To know a language is the ability to understand it from experience whereas to speak a language is the cabability to communicate vocally (acorrding to a rich vocabulary) whenever your vocabularies are much you're able to communicate people and at the same time you communicate when you are able to understand which means that the two actions are different but complementary. Different in the action and complementary in the function.
I hope the answer is clear
Interested in Language