Triangulation and social externalism
If we look at the starting point of learning a first language as involving (lots of) triangulation situations which are special in the sense that in those situations there necessarily is an asymmetrical relation of authority between the parties involved, is that not, pace Davidson, a form/source of social externalism?
If this is right, then one could make the case that first-person authority exists only as the result of successful communication. And that would make throw a different light on the appeal that Davidson makes to first-person authority in his argumentation against social externalism à la Burge.
The reason that Davidson wants to keep social externalism at arm’s length is probably that he thinks it might interfere with his claim that language is not an epistemic intermediary. After all, if we accept social externalism and accept the possibility of variety in sources of such external elements, then some form of relativism seem to ensue.