When I once encountered a homeless man writing sheet after sheet of incomprehensible scribbles while muttering incoherently, I could choose either to believe that
(a) his writing had a "key" of sorts, a Rosetta stone hidden from me that would provide a translation, or
(b) his apparent writing was not really writing at all.
To this day, both these alternatives seem live options to me, not because they are logically compatible (they are not), but because I lack enough evidence one way or the other.
In this instance, I waver before the choices.
I have little doubt, however, that either those scribbles are in principle translatable, or they are not, and if the latter, that the man was irrational, though not necessarily a danger to anyone else.
For his writing to communicate beyond some basic tonal level—something like a cry or scream—I assume a fairly high level of intentional transparency, even if he holds beliefs that are bizarre or false in my estimation.
I must understand how his writing can represent the world, and believe that the representation can be recovered.
The presumption of language—or the gift of language to another being—is the presumption of truth-preservation.