Simple World chat is a simple world chat management plugin. It keeps all the player's chat in the world their currently in. If there is no-one else in the world, it tells them "No one can hear you!".
There is also a bypass command "/chat bypass" so that people with the permission wordchat.bypass can chat in all worlds
The plugin's post on the Spigot plugin store: https://www.spigotmc.org/resources/simple-world-chat.5934/