The description of the zone of truth spell says:
you create a magical zone that guards against deception
But later on, it says:
On a failed save, a creature can't speak a deliberate lie while in the radius.
Say a character casts zone of truth on a mute person, or perhaps the party trickster simultaneously casts silence in the same area.
Under the mechanics of zone of truth, can an affected creature lie using sign language or a non-vocal form of communication?
Best Answer
Sign language will be at the GM's discretion.
As you noted, the effect of Zone of Truth is:
There are no explicit rules for sign language in 5e. It isn't provided as a language that may be learned, and it isn't incorporated into existing rules. There are some mentions of NPC sign language (such as Drow Sign Language). If your GM is allowing for sign language, you are in their territory.
Non-Verbal Communication May be Deceitful
The first sentence of Zone of Truth says:
This does not imply that it prevents every kind of deception. The specific effects are listed later ("a creature can't speak a deliberate lie"). The effect of preventing deliberate spoken lies is how it guards against deception.
Non-verbal communication may be deceitful. Zone of Truth regulates "speak[ing]". Non-verbal communication includes things like body posture, facial expressions, and other non-spoken contextual things.
If a regicide under Zone of Truth is asked, "did you kill the King?" they are not allowed to speak a lie. However, they can feign a surprised or innocent expression and body language to support it.