When I was young, I learned that saying "I don't speak Russian." is as simple as
Я не говорю по-русски.
However, is this polite? Would it be more polite to say something along the lines of "I don't know how to speak Russian."? The difference being that the first could be interpreted as not wanting to speak Russian (even when able to do so) while the latter makes it clear that it's about being unable to speak it. Is this a difference that matters in Russian?