The City of Silence
The youngest sub-city in Zil-Shahaar, The City of Silence is known for its rule of silence. A decree a few thousand years ago outlawed speaking in public, and has never been retracted.
Within a short time, several complex sign languages popped up, and while today's sub-city has a less complex web of languages, at least five groups of sign languages are currently in use.
Kids are gagged or magically silenced the first few years, then expected to be quiet on their own.