Truth telling ...

The rules for truth telling are whatever rules (tacit and explicit) that we follow to make conversation possible.

Sometimes, maybe generally, tacit rules can only be made explicit by changing the rules - even to the extent of changing what we mean by 'explicit'.

The open question argument guarantees that we can never make all the rules we are using explicit (in any intelligibly translatable sense).

A language for which the rules have been made fully explicit can be treated entirely syntactically - as s signalling system, or not a language at all.