Exactly.
Planescape's treatment of alignment exemplars aside, I really have never believed in an ecology or society, in the commonly understood sense, for demons, devils, and the like. They are corporeal representations of abstract ideals, and as such don't really require those things to fulfil their role. What is the family of languages that angels speak in the Judeo-Christian tradition, after all? Why can the devas and asuras of Hindu mythology talk to each other without translation problems?
I've always preferred the idea that outsiders and elementals, as somewhat abstract beings, communicate with mortals via translated telepathy; basically, the creature communicates on the purely mental plane and a mortal understands that communication as speech (with a colorful effect imparted according to the outsider's nature, and the mortal's perception of that nature). So a fire elemental communicating with a Red Wizard might seem to be speaking Thayan, but in a voice that sounds like hissing, crackling flames given speech. Perhaps outsiders do have a spoken language, but that language resembles the Dark Speech or Words of Creation; it is a tangible manifestation of power with its own concrete effects.