And some people say that just knowing one language makes you not a programmer.

I agree with that - but probably not in the sense as it was used in the discussion. Just knowing one language doesn't make you a programmer, just knowing 20 doesn't make you a programmer either. But you can be a programmer with knowing not more than one language.

Just knowing English doesn't make you a writer. However, one can be a writer even if English is the only language you know.