"Я знаю русский язык." and "Я говорю русский язык." is as I have understood it equal in meaning (please, correct me if I am wrong).
Today my tutor told me that: "Я говорю русский язык." is the preferred phrase to use.
I would prefer to use: "Я знаю русский язык" that is if they are equal in meaning. Let me know what you think and if there is a difference between these two.