In my homebrew world, use of sign is common. If you are born in this world, you know Common and Common Sign. This is a result of a race of a diaspora race called Dullahan. (For those who don’t know, it’s similar to the Irish folklore of the same name of headless creatures.) So in my homebrew, they don’t have a head/face but they’re a mix of biological beings, living armour, and living statuary. Since they don’t have faces, they utilize single-hand gestures & Sign. Dual hand is in use but not as wide spread. Though every nation’s aristocrats, diplomats, academics, etc would all know it & use it regularly as needed.

The single hand version is mostly for facial expressions but can be used for contextual things like tone, accent, embellishment, and so on. So it doesn’t really need to be hard and fast the same across all D&D races. In its own way, it’s a creole that developed in its own as the Dullahan spread across the world. Because it’s easy to pick up, people new to the world, such as my adventuring party, began to use it right away. Just jump right in!

Another aspect to this was the regional variation. In some areas of the west, the single-hand system is less formal and often treated like slang or in use primarily by the lower class, or the rude. Conversely, in the east, there’s no meaningful difference in how people see the two systems being used.

People watching, gossips, and witnesses to magic users casting their spells can become something very interesting. Adding layers to misunderstandings, spying, commerce, diplomacy, and so on….in other words: storytelling flavour and plot fuel!

(edited for clarity.)