Translation:Do you speak Swahili?
that has a different connotation to 'do you speak Swahili?', which is merely an inquiry, while 'don't you speak Swahili?' is understood as 'I thought/assumed you spoke Swahili?' To get the same tone across, you would say it as '(kwani) husemi Swahili?', which implies that the speaker assumed the subject spoke Swahili, but finds out that the subject does not. ('kwani' has a bunch of different meanings, in this case emphasizing the speaker's reaction to the subject's lack of Swahili speaking capabilities)