When you enter this
command into a command block, it will give you this error message, regardless what language you selected (because it has no translation)
- /scoreboard teams (join and leave) (uses the same method but does not print the correct error message in command blocks for some reason)
The following is based on decompiled version of Minecraft 1.9 using MCP 9.24 beta. All method and class names are the names used in the decompiled version.
The method net.minecraft.command.CommandBase.getCommandSenderAsPlayer(ICommandSender) throws a PlayerNotFoundException with this error message (not a translation) if the sender is not a player.