In English "to speak" and "to say" typically cannot be used interchangeably. You can speak a language but you cannot say a language. Similarly, you can speak with someone but you cannot say with someone (or rather, it wouldn't carry the same meaning). So speak is usually used to describe the nature of speech (what language is being spoken, who it's being spoken to) while say is describing the message being transmitted.
As far as I can tell, 说(话) seems to serve both of these purposes in Mandarin. My question is, are there any other words in Mandarin that can fulfill one role but not the other?