А кто сказал, что на парселе нельзя врать?

In an ordinary language, each participant in the conversation has his own mental mapping between sounds-spoken-or-heard and concepts-transmitted-or-received. Anyone who wishes to understand or speak the language must learn that mapping. If Alice and Bob have learned somewhat different sound-concept mappings, then the concepts transmitted by Alice will not match up completely with the concepts received by Bob.

Parseltongue is not an ordinary language. You either magically speak Parseltongue or you do not; you cannot learn or choose the mappings between sounds and concepts in Parseltongue. You cannot redefine a word in Parseltongue, so it is impossible to mean one thing by an utterance in Parseltongue and be understood as meaning something different.

At least, that is my theory.

Confirm.