Add a new config option that makes Essentials to use the language from the client(s) instead of one language for the whole Server.

You can use the Spigot method player.spigot().getLocale to resolve the client language or you could use the Protocollib packet PacketType.Play.Client.SETTINGS which is a bit faster.

Good idea, #2502 is actually currently already open for this but it's not clear that it will be merged soon.

Duplicate of #2029