There is a strange similarity in this discussion to the French discussion between "pouvoir" (kunnen) and "savoir" (strictly, weten, by extension, kennen), and that is perhaps not without its relevance, at least, insofar as Dutch is spoken in tri-lingual Belgium. The strict French understanding of "savoir" is "to know (mentally) how to do something", whereas the Belgian understanding is "to have the (physical) ability to do it" or "you can/could". Even in English, "can" is a subject of discussion, invoking such pedantic reactions as "Can you pass me the salt?" - "Yes, I can; the question is 'Will I?'." Thus, in France, savoir would be used in a phrase such as "Tu sais jouer du piano?" (Can you play the piano?) but not, as you might say in Belgium, "Tu sais me passer le sel?" (Can you pass the salt?). Whilst German and Dutch would be unlikely to resort to "wissen" or "weten" in phrases such as "Kannst/Könntest du mir den Salz geben" or "Kan je me het zout geven/Zou je me het zout kunnen geven", Belgian Dutch is otherwise frequently influenced by French usage, so that I wouldn't rule out the possibility that "kennen" and "kunnen" get melded in this particular usage of "being able to speak a language". Perhaps if for for no other reason than that the ability to speak a language (kunnen) is not per se a physical one (unless impeded by physical disability) but rather a state of knowledge, thus according more with the idea of "kennen". Even in English, it's not outrageous to say, instead of "Can you speak Dutch?", rather "Do you know Dutch?"